The warn level is just a guide for staff (and I guess yourself to see where you stand with us). The higher the warn level, the more (serious the) rule breaks you've made and therefore the more likely your access to the forum will be blocked. Over a period of time if a member doesn't create any further problems (that earns another warn) a member of staff will remove it. This is done manually and so can happen at any moment but as a general rule*, we usually don't reduce for at least 3+ months.
* To be more specific, none of us are able to remember who's been given a warning and when so we usually assess your warning levels when see a greater than 0 percentage next to your post (the more active you are the more likely we'll be assessing your warning level - if you got a warning 2 years ago and haven't posted since then it'll probably remain the same percentage).