Hey! It def depends on personal preference but we try to keep ours open the maximum (2 years). 

I like doing 1 year, so that guests who enjoyed their stay can rebook for next year. Make sure to increase next year’s prices (I use a setting in price labs to do this), so you can account for inflation.

I typically leave mine open for 12 months out. Although this totally varies, depending on market, some people do like to book that far in advance. 
 

I've never really seen a downside of doing this, outside of making sure that you're keeping on top of your pricing. 

The experts that I watch/follow are saying at least 12 mo. - I use 12 and considering going further out.

Usually 12 months unless there's some particular reason I have to only have it open for a shorter window.

I go with 18 months, which allows those ‘planners’ out there to book their vacations over a year in advance.   I use Pricelabs, and enter a 30% pricing uplift for anything more than 9 months out, in consideration of potential operating cost increases.   

I’d say less than 5% of my bookings are more than a year out.   Those who book more than a year out are typically booking for a big event, such as a big fishing tournament, sporting event or concert.