I'm the stepdad and I try and do 50/50 where possible. I try and do 'visible' tasks such as tidying up after dinner, folding laundry etc. because their dad didn't do any of it and I don't want them believing it's the woman's job.
They were 2 & 6 when I moved in, 5 & 9 now.
Financially I contribute slightly more to the joint household account. At first she suggested I don't pay towards the kid's stuff, but I pointed out that it would just mean I had a bunch of spare money that I'd end up using on them indirectly anyway.
What I can't take 50/50 of is the mental load. I don't have any legal responsibility so can't do much on that side (although the kindergarten gave me a login off their own back when they saw how much I was dropping off and collecting the little one, and more importantly how totally cool she was with it.)
We do tend to discuss decisions though, the dad is cool with it and there's a general understanding that my opinion matters but in an advisory way, rather than a casting vote, if that makes sense?
It works for us. The kids are with us for 12 days out of 14 so there was no point in going in half arsed.
I should mention, regarding household stuff - when I started seeing their mum she made it clear what she expected, even though much of it wasn't technically my responsibility. Because she needed the girls to see how an equal relationship should work. I was welcome to walk away at that point if it was too much.
The girls themselves have been fantastic about since day 1. They're awesome.