First, I don't believe that referenda should be held at the time of general elections, not at all.
Secondly, the time can be even shorter than that for general elections--30 days would be plenty. The main requirement is the technical one, how long it takes the election officials to get everything in place.
The third and final point is on this question about returning the money and so on, about excess funds at the end of the campaign. In considering that, committee members, I think it's important to have to relate that to other provisions that you may or may not want to address, such as spending limits, because they're all woven together.