No bad advice above. Well maybe one. Sorry Ralph, but it's not such a good idea to do multiple rounds of fill, sand, fill, sand. It is very difficult to get the micro mix the exact same density for each round, so the layers don't sand evenly, and the surface has waviness to block out later in the finish process.
Me? I'd sand the bonding surface thoroughly with 80 grit, after which I'd just slather on a thick coat of flox mix.
"Flox" is chopped cotton fiber. When added to mixed epoxy and cured, the fibers add tensile strength and toughness.
Add more flox thickness than you need so the addition is a one-time shot. Don't worry about how difficult it will be to sand away the excess, although if allowed to cure, a random orbit air sander will shape it just fine. The trick to to shape the flox with a
surform block or a
vixen file when it reaches the "cheese" state, well before full cure.
Use dry micro ("dry" meaning like firm peanut butter) over the newly shaped flox for final contour. If you apply the micro before the flox has fully cured (i.e. right after you rough shape it above), you won't need to sand the flox for surface tooth, as you would if you let it cure to rock hard.
BY the way, specific to your thickness question, micro is not like polyester filler, in that it doesn't shrink with time. That means there is no maximum thickness, assume you can cure without exotherm. I have cast parts in solid micro which I still have 20 years later...no cracks.