Off the top of my head, I remembered that the Lexus CT200h came with MXMs also. I did a quick Google and landed at a CT forum and saw that a majority of the posters in a thread about tire life said they got about 35k out of theirs. But of course, the MXM on the Model 3 could be a different tire (compound, tread, weight rating, etc)..
Personally, I would expect about 35-40k for a stock semi-LRR tire driving conservatively.. but I dont plan to drive conservatively