The thickness (or not) of a coat of resin will vary a lot, depending on who is applying it and how much pressure they tend to use, what formula of resin they are using, what the working temperature is and even how many minutes it has been since you first mixed that particular batch of resin. Therefore, applying a set number of filler coats and expecting the results to be the same from boat to boat is awfully inaccurate. Best bet is to determine how many coats to add by the actual results you are getting. The rule of thumb that we always used was to apply thin coats (they tend to drip and run a lot less, requiring less clean-up sanding once all the coats are on) and to roll coats on until the weave texture has totally disappeared. Then add one more coat as a sanding cushion, so that you won't be cutting into the glass during the final sanding. It's not unusual to end up with five or six thin, rolled-on filler coats, with new ones being applied as soon as the previous coat has stiffened enough that you won't disturb it with the roller. You may end up working around the clock for a day or so, depending on your hardener speed, but the time needed to apply a coat is only a few minutes, followed by some drying time before the next one.
No sanding is done between coats. In fact, it takes epoxy a week or so to harden completely to the point where the dust is no longer an allergy threat. If you're smart, you will give the boat a week or so to fully cure before you do any sanding on the resin, because you seriously don't want to develop that allergy. It would keep you from ever being able to work with epoxy again.
