Earlier quoted context omitted.
Four flips can get you 16 outcomes of course, rerolling values out of bounds means you need 4*16/10 = 6.4 amortized flips to generate a d10, or ~154 total to get 24d10. For large dice counts like that, it's probably better to just use a manual table for a bell curve based on ~8-10 flips. No human player is going to care about result fidelity beyond a thousand possible results anyway.
Actually I'm realizing a much more efficient (but still simple enough to be practical) way to generate d10 results is to do them in pairs. Do seven flips to generate a binary number to be interpreted as a "percentile" result in the range 00-99, rerolling on 100-127, then treat the two digits independently. That gets you to 4.48 amortized flips per d10.
Above 3d10 the rate of re-rolls increases; 20 flips for 6d10 gives a worse amortized cost than two sets of three.