The problem of flooding is not really an issue of time.
There are four known weaknesses of the PT-020 which you should be aware of:
- instances of the o-ring popping out from the corners and causing a flood
- not lifting the Mode Dial wheel of the housing can damage the Mode Dial of the camera during installation or removal
- the bulkhead (external strobe connector) is not very robust
- the polycarbonate 'bosses' or pins on the housing to which the port is attached to have been known to shear off if force is applied to the port
Keep the main o-rings clean and (sparingly) lubed and give the housing a good soak and rinse immediately after your dive. Don't over-work/over-maintain the main o-rings and keep an eye on them for stretching - when they do change them. If you're worried about flooding then get insurance.
HTH