How long can a cat go without a BM?

I adopted a new cat yesterday and brought her home at 2pm EST. She's doing very well and has adapted great, eating and drinking and playing. She's peed in her box, but hasn't pooped. I know she has to be stressed and there's been a change of food, but unsure when I should be worried. She has an appointment Saturday with my regular vet, but unsure if she should be seen sooner. Thanks!

Hello!  Im not sure what BM stands for - its not one we use in this part of the world - but I trust its the thing we call deafecation / poo / something slightly ruder.  If not, please let me know!   Anyway, you're correct;  this is not uncommon when a cat is in a new territoy as poo spots are extremely important in the feline world and it is very much not okay to poo in someone else's territory.   They may be looking for something unobtrusive to go, probably out of site, so its important to put lots of litter trays out in different places, ideally with different substrates, so that the cat can choose.  There are litter trays with a cover over them that appeal to some cats.  If they do get desperate and go somewhere you might not have chosen, its worth considering putting a litter tray there.  Another useful trick is to leave a small amount of their poo in a new litter tray where you want them to go.  It will then smell like a 'safe' toileting site.   However, as you suggested, the biggest concern is if she's trying to go and can't go, or if she's making herself uncomfortable.   Unproductive straining, or licking frantically at the bottom or belly, or even an abdomen that's obviously getting bloated with poo, should be treated as an emergency.  I would expect at least two piles per 24 hours.

However, cats can go over 24 hours when they can't find an appropriate site.