Title says it all.
Assuming a high enough mechanics level (minimum 3) and the Fast Learner trait , one could easily build 6 - 10 levels in mechanics in a single in-game day and less than 10 real world minutes by going on a vehicle repairing spree . I already have 200+ levels in mechanics within 2 in-game years, and that was only as a consequence of me responding to vehicle maintenance, not out of any desire to power level on my part. Compare that to about 21 levels in my electronics skill, which took nearly two seasons to build up and was a more involved process to build up by comparison.