In reply to Rollo:
If you can generate a tcx file from your device (upload to Garmin Connect, export, etc etc), then these can be opened in excel as an xml table.
I have my 310XT set to record at 1 sec. intervals, so each row on the table is 1 sec.
You can then count the rows per HR value to get number of seconds @ each HR, which is sort of what you want and doesn't take very long.
bit of a fudge, hope it is of assistance!
cheers
f