How often should I backwash my pool filter?
You can choose to backwash and replace the DE in the filter with new material any time you want. The increasing pressure readings occur because the DE filter material fills up with dirt sucked in from the pool. There are a series of tradeoffs that you have to evaluate when deciding to back flush or not. 

1) Increased pressure to pull pool water through the DE filter means that the pump motor is working harder and will pull increased current thus increasing the cost of operation.

2) More frequent backwash of the filter will incur additional cost of DE material to replace in the filter. 

3) Letting the pressure build up over longer periods of time can allow excess dirt to deposit in the filter. This can lead to caking of the filter material such that it will not flush put completely when you attempt a backwash. This will generally require opening the filter unit, removing the filter screens and cleaning them with a hose. This adds significantly to the work time required to do the filter cleaning. More frequent disassembly of the filter can also lead to higher chance of damaging a filter screen and requiring its replacement.

4) Depending on the sizing of the pool pump motor on your particular installation an increased pressure load on the pump motor will raise its operating temperature. In hot summer weather this can potentially lead to motor failure if it gets too hot. 

5) As you have already experienced, increased pressure due to a dirty filter, reduces flow through the pool circulation system. This can increase the amount of tine required to clean the pool if the water draw from the skimmer hose gets too low. 

If your time is considered even for the various tradeoffs then it mostly becomes a tradeoff of the electrical energy costs for operating the pump at higher pressure versus the cost and availability of the fresh DE material.