I count about 2s of isometric as 1 repetition.
Therefore, if a light day has say 10 reps, and a heavy day has 5 reps you will want about 10s hold and a 20s hold.
Obviously, where you are in a progression will help dictate what that may be. If for example, you can hold an adv. tuck for like 13-18s then 10s hold would work pretty well for that one.
FOr the 20s hold you can't do the adv tuck so you can do one of three things. You can add a weight vest or ankle weights to tuck planche. You can start out with adv tuck for 5-7s hold, then reverse to tuck for teh rest of the time. Or you can just hold the previous progression for an increased amount of volume.
Again, I will have some charts in the book to help determine hold times and such. This is the way I like to approach it though instead of massively increasing volume. Modifying positions to hold times, or modifying hold times to positions works best (just like you can modify loads to exercises or exercises to loads in traditional barbell work).