While TC visits are not allowed to be compensated, I usually provide some gas money, especially if distance is involved. As others noted, a nice trinket would be appreciated as well.

There is no min or max number of visits. 3 is a good min. After set up, about mid point after you’ve mastered your skills, and during the “systems” stuff.

Different TCs have different skill sets and experience. When we fill out application paperwork, we list our comfort areas. Fabric, wood, composites, etc. Some builders want scratch build tips, whereas others just want kit tips. I once had a builder get ticked at me because I didn’t know how to hand-form rib flanges. I’d only ever used flanging dies, and he thought I should know ALL metal skills. Not all TCs are A & Ps.