If he's only spending one full day per week on the bathroom renovation, then it's not any wonder it's taken 4 months so far. Have you asked him, how much longer it's going to take him?
Unfortunately, it's a project that he started prior to meeting you, and as you've only been together 4 months, you can't start dictating how or where he spends his time.
He can't stop renovating the bathroom, because he's already started it!! He needs to finish the job, regardless of your opinion.
I think you need to ask yourself, how much is your boyfriend still involved with his ex? I'd say quite a lot, if he's renovating her bathroom. Yes, his daughter benefits too, but she is 28, not 8! He didn't have to agree to do the job, so we have to assume him and his ex at the very least get along. Is his ex paying him? Or is he doing all this work for free??
You can end a relationship at any point and for whatever reason. If this isn't working for you, then end it. By the sounds of it, your boyfriend doesn't really have the time for a girlfriend.