I ran a poll once
to see what most people thought would be a reasonable time frame for voting. Most people felt that a week was fair (traditionally, the first round tends to last longer than that, while people get a feel for the game and write up stories for their "characters").
Like I said, for this being your first time as Mafia game GM, I think you handled it quite well. I would much rather have a GM who's too pushy than one who lets the game stall out and go inactive.