When do you flush a DE filter?

[waterbear]

I will always recommend taking a DE filter apart and manually cleaning it at least once a year - it lets you check what's going on in there

At Least! Personally, I feel a DE filter should always be broken down for cleaning and only backwashed in situation where you are filtering a lot of gunk (dead algae, phosphate remover, etc.) that causes a fast filter pressure rise repeatedly and then, once the problem is solved, it should be broken down.
The reason is that you never quite know how much DE you are flushing out when you backwash so you never REALLY know how much you need to recharge which can lead to an undercharged filter (and fouled grids) or an overcharged filter (and DE bridged between the grids. In base cases it needs to be chipped out with a screwdriver or something similar!)