How long do they last in the cache in the DJI GO app?

Sometimes I get to the launch area and the map isn't there and sometimes it is.
Why is that?

I am using an iPad Air 2 w/cell (for the GPS only).

I use an Android and I use Litchi.  For me, the cached maps last something like 30-45 days.  I'm not sure of the exact number.  I guess the tablet's OS considers them a temp file and so wipes them out when they expire.