This is actually a pretty good question. I think everyone in DG has the desire to constantly try new molds and see how they stack up against what you currently bag.
I generally bag discs based on the following:
How consistently I can put the disc on the line I want. I don't want to throw a disc and have it do different things every time I throw it. This mainly comes into play with stable or understable discs. I don't want a disc that randomly flips at the wrong time.
How readily available the disc is. Just about every disc I bag I have backups for. I don't want to throw something super rare that's impossible to replace.
How a disc performs in wind. This is very important. If I'm looking for an overstable fairway or distance driver I don't want a disc getting flippy in headwinds. Same goes for downwind. I want something understable that will flip up and ride nicely. I also tend to stay away from mids that get tossed around too much by wind.
What plastic a disc is made of. I bag premium plastic. I don't like trifling with pro, DX, or X. I think that plastic gets destroyed too quickly and it isn't worth the hassle.
Sheer distance: This isn't the #1 thing I look for in a driver, but it is still important. For an open hole you want a driver you can absolutely lean on for big yards.
I don't change my bag up often. A disc has to absolutely blow me away on the course in a tryout for me to start bagging it. That said, if I notice I'm consistently throwing some mold in my bag poorly I'll start looking to replace it. Most of the molds in my bag have been there a long time.