How much time do I need to spend in Myanmar during a border crossing at Mae Sai?

I did the same, but found it a bit pushed time wise. You can encounter road works and numerous police stops on the way up and coming back into Mae Sai from Tacheleik there was a bit of a wait in a queue. We made it back to the Mae Sai bus station with 5 mins to spare. It was stressful doing it in this time frame and a quickly gobbled meal was hardly enjoyable but otherwise we would have been ravenous during such a long day. If I did it again, I would go earlier and allow myself more time over in Tacheleik