In theory, you should be able to plan and submit up to 5 days in advance, but if the route is not available on the day it is beyond all but Eurocontrol's control. Many airlines use RPLs that are planned months in advance, so they must find a way around it.
And there was me standing next to a chap in the Eurocontrol facility at Brussels, in the actual room with "IFPS" on the door, with him telling me how much hassle they have with airlines submitting repeating flight plans, which work on the Monday, and break on the Tuesday, and IFPS staff then have to manually fix up the routings on Tuesday, Wednesday, Thursday, Friday, Saturday.... they will have told the airline to fix the routing by about Wednesday and by the weekend IFPS chucks the whole thing out and the airline's OPS department has to produce a routing which validates on the Monday again........ and then the process repeats
Fixing broken routes is a marvellous job creation scheme at IFPS, but one can bet that the one time you really are up the **** creek and really want the RMK IFPS REROUTE ACCEPTED to work, IFPS will wash their hands of it (as they are absolutely entitled to do).
I am no "sky god" but having been caught by this a few times I would never file a flight plan 5 days in advance and
a) expect the route to still validate when I change the DOF/ after say 3-4 days (especially if I change the DOF/ between a weekday day and a weekend day, which changes many airways)
b) expect to be able to depart as planned after such a long time (weather etc)
The only way to work the IFR system reliably is to be able to, on the day of the flight or shortly before, develop a valid route, and file the flight plan with it. Anything else is a bonus, and often breaks.
That is why a well usable mobile-internet application is desirable. It's not hard to do, at all. It should have just been in the original specification
You are nearly there now; you just need to sort out the version control and publish a fixed update schedule, say 1st of each month,
not force a download of the update for the whole of the following month, and for each update publish which features of the previous version will not continue to work after the update date (for people who are unable do do the download when they are away). Downloading 4MB is trivial
if people know when they need to do it.