How far in advance can you file a flight plan through Sky Demon?

There does not seem to be a restriction in SkyDemon. Its rumored that that the ATFN system doesn't like plans submitted more than 5 days (I think) in advance though.

The advice for filing a FPL more than 24 hours in advance is to add the text "DOF/120914" for example to indicate the flight is to take place on the 14th September 2012 into the box 18 details.

Though I noticed that SkyDemon doesn't seem to do this - or at least its not visible in the information presented to you immediately prior to submission.

Many thanks that is very helpful

AFPEX sets a limit of 10 days I think.

For a VFR flightplan you will find this limited more by the staff at the receiving end, who are often dealing with a piece of paper that comes out of a printer. This piece of paper might end up in the bin if filed too far in advance. There is not much you can do about that. Normally filing 24 hours in advance works well in terms of both machine and human factors.

Used the flight plan functionality for the first time recently on a trepidatious flight over the oggin complete with jackets and raft. Quite marvellous. 50p per plan is good value for a great service. Very easy to use and it seemed to go through the system effortlessly. Well done.