2 nights.
2 bus companies have trips from Montevideo to Porto Alegre. Both are about 70-80 dollars, 12 hrs.
http://www.ttl.com.br/
http://www.egakeguay.com/index.php?ega=linea_regular&op=horarios
I will probably couchsurf since I cant find any good hostels. There is a really nice looking girl named Bruna.