What should be a typical time horizon to get in and get out of leveraged ETFs
Get in when it is about to go up. Get out when it is about to go down. I'm not trying to be a dick, that's about the only good advice on the subject I can think of.
"Time decay" refers to the fact that these funds borrow large amounts of money and/or invest in derivative contracts that expire on a certain date. This has the effect of boosting volatility relatively cheaply (which is why most customers buy in the first place) but the passage of time has an inherent cost.