If you're bumping the DE, you shouldn't be going through more DE I don't think; I don't use a DE filter, but I understand you only have to add more DE when you backwash the filter, or otherwise clean out the old DE; bumping just stirs up the DE so a different "section" is being used as filter...
a full filter shouldn't make you lose prime- it'll build up pressure, and restrict flow to the return, but your pump should still have water at the impeller with nowhere to go.
Losing prime is usually a leak somewhere (suction leak maybe in pump basked lid or suction piping, or clog at skimmer?)
As far as how long- it varies based on size of filter, size of pump (how much water is going through it), and how cruddy the water is... with dirty pools, and especially if filter smaller, an hour is very reasonable to clog as it removes the crud from the pool