@seedygreenute, thanks for the kind words. The general rule with bolts is that you want the bolt to engage enough threads to be equal or greater to the bolts diameter. So for a 3mm bolt you want to engage at least 3mm of threads. If you have a look at nuts you'll see the commonly available ones follow this rule unless space is at a premium for whatever reason or the material you're threading into is really soft like plastic then you want a longer bolt with more threads engaged.
With a typical 3mm motor screw/bolt you want that 3mm minimum plus a little extra. Threads don't exactly start at the top, plus they're going into aluminum. So let's add 2mm for where the threads start and another 2-3mm for the fact we want a few extra threads for going into aluminum and we can round up to an even 10mm needed sticking out the motor mount to thread into the motor endbell. This will engage enough threads and shouldn't be so long that the screw will contact the windings on motors that don't have protection from that happening.