If your goal is to work professionally as model, TF* or testing is a means to a goal. The point of testing is to create portfolio worthy images for use in your book and online portfolio so you can get more paid work. If you have more than 20-30 images, you probably don’t need any more. It’s a good idea to update your book every quarter or so, but you should be able to update your book with pictures from your paid assignments.
Aspiring models hoping to get signed or book work directly should always “test up”. This means you have to work with people who can bring your work to the next level. Shooting the same pictures with a different photographer in a different outfit and location is a lateral move, not an upward move.
Valid reasons to testing include portfolio test and client test to be considered for a future assignment. Testing is not about charity. It is rarely about getting practice. Obviously, there are many models who just want to model for art, fun, or to get pretty pictures. For these models, the act of modeling may be more important than the pictures. If you are one of those models, go nuts and shoot TF* as often as you like. For aspiring professional models, be selective how often you test and with which photographer. It may end up hurting more than helping.