We do ours a bit differently than those above, but it takes about an hour to run off and last year raised over $4,000 in a community of about 1,000 people.
We make a big deal out of it and advertise it on the radio. We also invite "celebrity" hitters to increase attendance (DJ's from local radio, sheriff, principal, ag teacher, etc.), and we have it at night under the lights.
We send our players out with "pledge sheets" about 2 weeks ahead of time and tell them we expect the to raise no less than x-amount of dollars. They will take either flat donations (ie, $25) or pledges on how far they can hit a baseball. Some people don't understand the $-per foot concept and just want to donate a set amount.
On the day of the H-A-T we set out cones in the outfield in an arc at 200' to speed up measurement. We measure total distance of the batted ball, including roll (it IS a fundraiser, after all) and use the longest to tally up the pledge sheet. We don't have a set number of swings, but usually within 12-15 a player will get all of one and we measure. I'm in the press box with the players pledge sheets and when my assistant gets a measurement he relays it to me and I announce it over the p.a. system and total the players sheet as the next hitter moves into the cage. Our players rotate from the shag bucket, to feeding the machine, to on-deck, to hitting.
So the actual process goes like this: I approach you and explain who I'm with and what we are doing to raise money. I tell you how it works, for example, if you pledge 1 cent per foot and I hit it 400' you owe me $4. You decide to donate a dime per foot. On the night of the hit-a-thon my farthest batted ball traveled 425', you owe me $42.50. I return to you in the following days to collect and turn it in to the coach by a pre-determined date. If I'm late the Coach has a "reminder" for me that doubles in number every day I'm late.
Once again, the best part is it takes about an hour (for 20-30 players) to run off and the only work I've had to do is run off pledge sheets.