I'm not sure which brand I took, but I have on and off used the preventative inhalers since my early 20's (i'm now 41). I use them when I'm having a particularly bad time with my asthma for whatever reason (allergy season, pregnant, wrong country etc.). I generally find that a few weeks to a few months will do it and as my asthma improves I have either weaned myself off slowly or stopped cold depending on why I was using it in the first place. I haven't found much difference in either scenario if I'm logical about how I do it. For example, at the end of the summer in london I was having really bad asthma and started using my preventative inhaler twice a day. We then went to italy for 2 weeks. I stopped immediately when we got there as I've never had allergy problems in italy and I was fine. The weather had changed by the time we came back to london and I didn't need to start again when we got back).
Hope that helps a bit. Your gp should be able to advise you on how to wean yourself off.