When and what to increase?

I am routinely hanging 5lb X 4 sets X 20+ mins Per set with heat each day. I’m starting to find it more and more difficult to reach an exhaustion point. This is especially true since I read BG’s thread on fowfers and began sleeping tucked. I’m noticing nice gains, especially flaccid, and I want to safely keep my progress going.
So the question is: Do I add weights or sets?

Thanks in advance gentleman.
Keep growing!