Just till it's dry to the touch (not tacky) ... if I feel the slightest tug, I know I messed up the surface and I need to do a quick touch up sand (wet 1000 grit) and apply a few more layers.
The thin CA likes to fill in 1000 grit (and similar grain size) scratches quite effectively. Your 1500 grit would work too, but I think 2000 grit wouldn't be quite aggressive enough in getting the major defects removed. Anything rougher than 500 grit would probably remove way too much way too fast.
Even if you have 12 layers built up, they are very thin ... remember that most of your CA actually ends up soaking into your applicator pad, unless you are using a craft foam pad or a little pen parts baggie, which won't soak up CA ....
If you use that method, you will want to use MUCH LESS CA per application.
I also use BLO (boiled linseed oil) just before the first application of CA ... helps to make it go on even smoother. My BLO comes from Minwax Stain'n'Seal (natural amber color). Helps to make the grain REALLY pop, and the CA application freezes it like that.
This is a picture of my Samples box. You'll notice that I turn a lot of blanks round, and then sand, finish, and polish them while they are still full length blanks. This helps my potential customers see what a finished pen may look like, and also helps me to streamline the process of making a pen ... it's already round, I just need to chuck it, cut to length, drill, ect ... I turn all my barrels round before cutting and drilling, so this does save me some time as well.
You'll see the finish results I get here from merely sanding up to 1000 grit (dry, bare wood), cleaning the wood (you see I didn't clean the one third from the left at all, I was in a hurry), applying my finish (minwax + ONE COAT of CA), wet sanding at 1000 grit (very lightly, under 5 seconds), polish with Plast-X.
The beads in the top right are all off-cuts from Breast Cancer Awareness pens.