Place the Mi24 where you want it to start, and make sure the special field is *not* "flying" since you said you want it to start its engines and lift off.
In order to keep the Mi24 on the ground until the desired time, I usually make the pilots disembark and wait on foot. Supposedly, the land command will make a helicopter land and shut off its engines, too, but I haven't experimented with that. So, using my method, make the following waypoints for your Mi24:
WP1: get out. Put it near the Mi24.
WP2: move. This should be relatively close to WP1. It just moves the pilots to that spot.
WP3: get in. Put it very close to the Mi24, or even on top of it.
WP4 and greater: whatever you want the Mi24 to do.
So, the key to making the Mi24 wait is to create a trigger and synchronize it with WP2. In the trigger's properties, set the count down times (low, mid, and high) to however long you want the Mi24 to wait before it takes off. Make all three values the same, unless you want the time to be random.
Thus, what happens upon mission start is this:
1. Pilots disembark from the Mi24.
2. Pilots move to WP2 and wait until the specified time passes.
3. Pilots board Mi24.
4. Mi24 takes off and follows the remaining waypoints.