Thailand Visa on Arrival Duration

I can understand your confusion with seemingly official sites (re UK):

Visa on Arrival

Passport holders from the below 28 countries and territories may apply for visas at the immigration checkpoints on arrival for the purpose of tourism and will be granted to stay in Thailand for a period not exceeding 15 days. The applicant must provide proof of means of living expenses at the amount of 10,000 Baht per person and 20,000 Baht per family accordingly. The applicant must possess a passport with at least 6 month validity and must present full paid onward travel tickets which are usable within 15 days of the date of entry. Visa on arrival can be applied at 24 designated international checkpoints and applicants should submit a completed application form with a recent photograph. The application fee is 1,000 Baht. Visitors who enter the Kingdom with Visa on Arrival generally cannot file an application for extension of stay, except in special cases of illness which prevents them from travelling.

And yet elsewhere on the same site:

Nationals of the United Kingdom and over 40 other countries are eligible to travel to Thailand, for tourism purpose, with the exemption of visa and are permitted to stay in Thailand for a period of not exceeding 30 days. Therefore, you do not need a visa.

Even here is not too precise at one point:

They must have a confirmed return ticket to show that they are flying out of Thailand within 30 or 15 days of entry, as appropriate. Open tickets do not qualify. Travelling overland out of Thailand by train, bus, etc to Cambodia, Laos, Malaysia (including en route to Singapore), Myanmar, etc is not accepted as proof of exiting Thailand. You may be asked to show your flight ticket on entering Thailand. If you do not possess a flight ticket to show you will be exiting Thailand within 30 or 15 days of entry you will be most likely to be refused entry.

but I think fully explained elsewhere. New Rules for Visa on Arrival

30-day Visas now Down to 15

Border runs have been cut short due to the very recent change in rules regarding overland entry by Thai immigration. Foreigners entering Thailand via border posts at Cambodia, Laos, Myanmar, and Malaysia without securing a prior visa will now be granted only 15 days of stay in the country as opposed to the previous Visa on Arrival, which was valid for 30 days. According to a spokesman for the Thai Immigration Bureau, the new regulation was created to encourage foreigners to secure the proper visas in advance from a Royal Thai Embassy prior to their arrival. The new rule will also limit the amount of back-to-back visa runs being made by foreigners to extend their stay. However, this change only applies to visitors who have not obtained a visa beforehand, and are entering from the border by land. Those who arrive at the airport without a prior visa will continue to receive a 30-day stamp. Extensions at Thai Immigration after the Visa on Arrival has expired remain at 7 days, after which you must leave the country or pay the penalty and other sanctions for overstaying the visa. The only exception to the new regulation will be Malaysian passport holders traveling from Malaysia, as they will continue to receive the 30-day Visa on Arrival. Immigration authorities further clarified that there is no longer a limit on the number of times a foreigner may enter Thailand within the days allowed on the Visa on Arrival. The former provision stating that visitors cannot stay for more than 90 days within six months has already been dropped.

The key element from the last may be "Those who arrive at the airport without a prior visa will continue to receive a 30-day stamp."

If you are a citizen of countries such as UK, USA, Canada, Australia, NZ, EU member nations, then you may qualify for a 30 day entry stamp (aka visa waiver entry). To qualify, you need to 1) document that you will depart Thailand before the 30 days are up (confirmed airline booking); 2) prove financial status (10,000 baht for one person). Thai immgration rarely checks these two, BUT they can as I have personally witnessed people being asked. Your airline will confirm item one before they let you board. To stay longer a 60 day tourist can be applied for in advance.

If you are a citizen of countries such as India, you can apply for a Visa On Arrival which gives you 15 days. Or you can apply in advance in India and receive a visa for 30 days.

Visa waivers, VOAs and tourist visas are all single entry and expire when you depart irregardless of how many days you used. Travelers qualifying for visa waiver entry can get a new stamp each time they enter, though immigration officers now have powers to deny one if they feel you are abusing the system and staying too long in country. Folks traveling on tourist visas need to arrange multiple visas to reenter.

Visa waiver entries and tourist visas can be extended once for 30 days once in Thailand, but this ability to extend does not negate the requirement to prove you are leaving before the initial 30 day permit.

All of these rules are at the discretion of the immigration official, they can shorten your permitted stay if they feel it warranted. These rules can change without notice, as Thailand been frequently adjusting its rules to encourage tourism while trying to weed out expats who are flying under the radar.

Still the same. You'd be issued a 30-day visa-free stay upon arrival. I think the 15 days should only apply if you arrived in Thailand, by land, from a neighboring country. You could also apply for a single entry visa in your home country, allowing you to stay in Thailand for 90 days.