It doesn't have to be full-time work (part-time is OK) but all the work has to be documented (i.e., in a log) and signed off by the supervising A&P and/or notarized by the employing company.
Minimum 18 months = 2,880 documented work hours for either the Airframe OR Powerplant rating.
Minimum 30 months = 4,800 documented work hours to be eligible for both ratings.
The work experience must cover at least 50% of the subject areas listed in Part 147.
Obviously if you don't do it full time (either through work or school), then it will take many many years to get the required experience.