I have looked at this and my instinct says that setting it to a longer time increases the risk of crashes in the event of a power failure but may speed things up marginally or possibly may just speed up other things writing to disk if running disk intensive programs. Is there anything else I need to be aware of before playing with this setting?

All of the CPDN applications perform their checkpoints at fixed points (every model day for HadAM3P and every 6 model days for HadCM3), so the setting has no effect on the project. Some other projects have similar restrictions either across the board or with specific applications (e.g. WCG's CEP2 and FAHV applications have fixed checkpoints but the other applications use the preference setting).

Some projects have applications which write a lot of data during checkpoints and I remember one case where a user was finding no progress being made on a slow computer because he'd set the checkpoint interval shorter than the time taken to write the checkpoint data!

I'm not sure what the default checkpoint interval is (possibly 60 seconds), but 600 seconds works well for me.

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"The ultimate test of a moral society is the kind of world that it leaves to its children." - Dietrich Bonhoeffer

I would like to know what is the time interval between checkpoints for the hadam3p WU...

I had accidently shutdown windows without suspending projects and have had data corruption for hadam3p projects...

I think that the time will depend on things such as computer speed, etc.

The time of a checkpoint should be part of the info under the Properties button.
Go to the Tasks tab, click on a model name, then look down the list of buttons until you find the one labelled Properties. You may have to do some maths.