One or two days, either is OK with me. However, due to traveling time, tournament fees should include a hotel room for 1 night more then the actual tournament lasts. Example: A tournament held over 2 days should include 3 hotel nights.
That having been said, I think a tournament held in just 1 day is more desirable because it attracts more players simply for the fact that it can be squeezed into a weekend (without too much difficulty) without taking any time off from work.