Hello
Give yourself at least 25-30 days in Bangkok before you add in any sightseeing. It's certainly worth planning a few days of sightseeing before you get into Chett's rather challenging liquid diet and cleansing routine which will consume around 4 days and could leave you feeling a bit week.
My feeling is that 25 days after the surgery are more than enough. Chett seems to get things right (or very close) the first time, so as long as you listen to Sri (his brilliant head nurse) and take things easy, you will be more than ready enough to fly.
On the subject of flying, do try to get a direct flight home if possible, and certainly a seat you can recline almost fully. This will allow you to avoid putting the external stitching under stress, and it is a lot more comfortable if you find sitting a bit of a challenge.
And enjoy the experience - Chett and his team were fantastic during my visit: serious, kind, gentle, massively experienced, and very friendly. We often had tea when they came round for the daily checkups and it made the day so much better for all of us!
Regards
J