Staying full six months using UK Standard Visitor visa

You have clarified in comments that you are a Phillipines citizen (Filipino) and as such, according to the UK government, you will need a visa to visit the UK for the intentions you have outlined.

The visa you require is the Standard Visitor Visa, which typically is valid for 6 months and covers visiting family and friends. This validity period is typical regardless of the period applied for, and as such can tempt visitors to stay longer than they apply for, or understate their intentions.

There are two things you may fall foul of - firstly, if you intend on staying the full 6 months, you must say so during your application. If you fail to state your true intentions, and stay for longer than you stated in your application, then you can at worst fall under deception rules for subsequent visa applications or at best it can damage the credibility of subsequent applications, so you need to be up front about your intentions.

Secondly, you could find yourself falling foul of the "attempting to live in the UK through multiple visits" rules if the ICO thinks that your multiple extended term visits constitute such an attempt.

Above all, just be truthful and present a good case.

As part of that good case for a visa, you must convince the ICO that you intend on leaving the country at the end of the visit - this will be difficult for a 6 month stay, as it indicates you do not have good ties with your current country of residence, and indeed it shows that you have good ties with the UK, so proving you wont overstay may be difficult in your situation.

Personally, I doubt that it would harm your application to state your ongoing intentions with regard to the relationship, especially the steps you intend to take if your relationship is successful - this may indicate to the ICO that you have an understanding of the rules in place and may increase the credibility of your application if you are forthcoming about everything.