If you go to the track twice in one year with a full tank, with it spraying on 8 (using Alky-Control kit) you will probably have from the top of the tank (which is the overflow tank) probably go down 2 inches from the top.
If you dont go WOT on the street or push more then 10 psi on the street, then it will stay at that level or a little below because the alky will not spray until you go above 7 psi or so.
For me, if i go to the track and make 1-2 passes, i can about 1 month without filling the tank since im not at WOT all the time on the street, but i do get around 20 psi on the street sometimes. Also i fill the tank when ever i pop the hood and think about it.
If you dont get into it on the street, your alky will not go down because it only sprays when you go above X amount of boost.
Hope this helps,
Adam