Depends on preference obviously but I tend to go by the size of the bubble bar. The standard size (French Kiss, Vesuvio, Mrs Whippy etc) I'd say you get two decent baths or one luxurious one.
Larger bubbles (Comforter, Lemon Crumble, Matcha etc) I'd say about 4 but you could stretch them out further or have 2 really fancy baths.
Reusable bubbles last for ever, even the smaller ones.
I usually get quite alot of baths out of my bubble bars. I chop mine into a crumble and keep them in their own jar.
I then tip what I need into a seive and run it under the water. It creates so many bubbles this way and I've only used a bit of the bubble bar.
Depending on the size of the bubble bar to how many baths I get out of each one but id say with the smaller bars I get maybe four. If it's one of the large ones like the Comforter then I'll get probably double that 😅
The seive definitely helps especially when the bar broken down into a crumble.
I used to use them whole! Or bigger ones in halves. Now I probably go halves or thirds on the smaller ones and even smaller again on the bigger bars. Comforter probably does 4 baths for me, or maybe 3 indulgent ones! I love lots of scent and bubbles tho. The Mother’s Day jasmine cream and violet cream bars have been amazing tho... I’ve probably used 1/6 each time and the baths have been amazing!!
We have really hard water in Bristol (UK) so annoyingly they don't go as far. Smaller ones I split into 2 and bigger ones will be 3/4. When I visit home and use the same amount though the bath overfills with bubbles 😂 I also have this 'bath tea strainer' from the bodyshop a couple of years ago which is amazing... It just looks like a bigger loose tea strainer on a chain which is great because you can crumble your bar into that, hook it around your tap and just leave your bath to run - basically the same a seive but more convenient and measures it out for you. I'm not sure if they still sell them but would be awesome if lush made something like this!