So, for brisket, you usually can plan on 1.5 hours per pound for standard choice brisket (this can vary for various grades as well, down to 1 hour per pound) but, for some reason, in my experience at least, it takes much longer to get to temp the smaller the size of the cut.
You also will run into a stall, which varies but can usually show up at the 165* range and take a while for it to really get past that temp. Hopefully that brisket cooks for you ok, but another tip to help it, you could wrap it in foil or butcher paper to help it come to temp a bit quicker. But, try to only do this once it starts to climb out of the stall so you don’t loose that momentum the brisket has started to gain. If you use foil, you may lose a bit of the bark formation because of moisture trapped, that’s where butcher paper holds an advantage, allows some of that moisture to escape.
another tip, you could kick up the temp a bit, to 250*, that could help it as well. I have started to cook all of my pork and beef at that temp now, provides great results and doesn’t take quite as long as 225* - I’ve even seen others on this site that cook at 275* with great results.
Don’t give up on it though. I think everyone who smokes bbq has ran into a situation like this that altered plans for dinner - so don’t sweat it. It will be great!