I don't wash mine. Seriously. I just keep a spray bottle filled with 2 cups distilled water/ 2 tsp TTO in the shower and spray the liner after the day's showers are done (although we usually only shower every other day). About every 2 weeks, or whenever I think about it, I stand on the outside of the tub and push the liner back away from the tub and spray down in there, where mildew likes to try to sprout. The TTO totally kills the mildew and just keeping it sprayed off after the showers has kept my liner clean since we've had this house (about a year, so far!).
Just my 0.02.
Darci