We have a Vacation Savings Account for this.
Vacations (to disney or elsewhere) are pretty much our familiy's entertainment. So it has a big line item in our monthly budget (seperate from regular savings etc) We generally know in advance the approximate time span we'd be going on vacation and I back up from there and determine the "min" monthly deposit in order to cover the cost of the vacation (and I always estimate high when determining the cost of our vacations) and I put that amt of $ into our vacation savings account. In addition, 1/2 of any "found" money goes to our vacation savings account (bonus, refunds, item sold) When I have enough in the vacation savings account, I book the trip. My 1st Diseny trip (Nov 99) I used a credit card for and when I got home, I really DID NOT LIKE getting bills for something that was already over & done with so we said, never again. since then, if the CASH is not there, we don't go, or if enough cash is not there, we adjust our vacation plans accordingly. This works for us.
as far as how long does it take, that's very subjective. Some families have a lot more discretionary funds left over after paying for necessaties each month. as an example, DH & I bought what was to be our "starter" home in 1996. over the years, we deceided not to "upgrade" as most families in our area tend to do. As a result, lower housing costs allow us to put a higher amount of our discretionary funds into a vacation/entertainment fund vs pumping it into a larger mortgage. Nothing wrong with the larger mortage, this is just what works for us. We can easily save several 1,000 in a few months time (again, seperate from regualar savings) without feeling the pinch at all. we feel very fortunate to be in this position.