In my opinion, anything on a product backlog which exceeds a 2-3 iteration horizon should be spun into a product roadmap document instead - as a general rule of thumb, if you don't have enough information about a story to sit down and write a clear set of acceptance criteria for it right now, it probably doesn't belong in your product backlog.

2-3 iterations is doable - trying to maintain a backlog of stories 2-3 releases ahead usually results in a big sloppy mess of a backlog.

Unfortunately, I've worked with many clients where thinking about priorities in the future far enough to have 2-3 days worth of stories in place beyond the current iteration is a struggle. This is where things usually go really off the tracks, because you build the thing it's easiest to describe right now to keep everyone working, or run from externally-directed fire to fire, instead of setting a cadence of development which represents the business' true near-term priority mix.