Is it bad to leave your syringe sitting for multiple hours containing meth and your own blood that got in there earlier that day then injecting it?

Yes its very dangerous and not worth it. When blood sits in the syringe it starts to coagulate (clot), this can cause arteries to clog, abscesses and cotton fever (and other problems). Bacteria is a whole other problem which also has a list of health concerns on its own. You don't want blood to sit in the syringe for more than one minute and after a couple minutes your better off just eating or blooping the hit. If blood has been in the syringe for hours I wouldn't even eat or bloop it due to bacteria growth. Don't be desperate and just toss it, its not worth the severe consequences.