The maximum you can theoretically earn are 135 experience points per day, so for example to get from your level 5 to level 6 is a 1000 experience points so can be done as fast as 8 days. The maximum points requires some genuine creative effort to create a thread which is sticky. Most days I'd expect you'd be earning around 70 experience points so that would take 14 days. So 1 to 2 weeks, assuming you engage daily.
Up around the level I am at to move a level is 12000 points so takes 6 months.
To get to the highest level, well it is years. e.g. the highest Level 21 at the theoretical fastest takes 5 years but more realistically 10 years.