I was just thinking about giving my baby a bath and it made it wonder, but when is a good time to switch to an actual bath tub?

I'm still using her baby tub and occasionally I'll take a bath with her in the big tub. I plan on getting a non slip mat whenever I start to put her in it. She's sitting up very well, crawling, trying to pull herself up on things to stand (just thought I would include some milestones just in case that kind of determines when she is ready). Also she is 8 months old. Thanks for any advice!