What is an acceptable time to wait for other passengers in a songthaew in Thailand?

Today I was taking a Songthaew (shared taxi) in Krabi Town and there were only three passengers, me and two friends, present. The normal fare after 6pm is 60 baht.

We were offered to start the ride immediately if we paid 40 additional baht per person (100 instead of 60) or to wait until three more passengers would arrive. We declined the offer assuming that we would not have to wait long.

Nobody arrived for ~20 min and I asked how long we would wait for more passengers. I was told that the shift ends in 1.5 h and that is how long he would wait. After additional ~15 min I agreed to pay the extra and we drove off. We were asked to pay the money in advance, usually passengers pay at the destination.

1. What is the socially acceptable time to wait for other passengers?
2. Was the long waiting time (~35 min) and the requirement to pay in advance a show of disrespect/distrust?
3. Is there something I could have done better, assuming that no immediate alternative in this price range is available?

Additional Info:

• from Krabi Town
• to Ao Nang
• in June, off-season
• there was a guy sitting with us in the passenger area but who was not the driver, he was the one I talked to, and the one who collected the money and gave it to the driver upon departure, he did not come with us. The driver did not seem to speak English.