I found this helpful..
Because wear and tear vary greatly depending on use and environment its good to do a visual inspection regularly. I check the line every spring when the studio warms up enough to use and every time I swap the tank. it works out to about once a month.
Though usually its because the grill is empty I'm not actually using that much propane.
http://www.glassalchemy.com/welding_hose
Hoses usually show more wear near the fittings or the last 4' of hose before the torch.