Well,a weak condensor can appear to mess up pretty regularly. I believe if you do a search, you'll see that Ron Brown's MBK condensor seemed to fail regularly around the 20 minute mark. Also we had a coil on a riding mower that would give you 10 or 15 minutes and quit every time.
Also, depending upon the level and amount of fuel in the tank, it CAN wait easily a half-hour because of a plugged gas cap vent. 45 minutes into my ride one day going to work, my fuel stops flowing. Thereafter about 20 minute intervals and usually when I was going at least 30mph, it stopped flowing again.
If you have little fuel in the tank it's gonna take longer than if you have a full tank.
Fill the tank,tighten the cap,and take the float bowl loose with the fuel turned on and see if it flows well thru the needle valve. See if it slows down after about a pint runs thru it........(yeah....that's a lot)! You did clean the banjo filter where the brass screen is on the carb,right?