This article is from the Recreational Figure Skating FAQ, by Karen Bryden with numerous contributions by others.
If you're skating only a few times a week, every six weeks to two
months is probably frequent enough. You should get in the habit
inspecting them each time you dry off the blades, and when there are
dulled or there are a lot of nicks, get them sharpened. The usual test
for sharpness is dragging a fingernail lightly over the edge - if it
planes off a little sliver, they're sharp, if it just slides, then
they're dull. It is not unusual for blades to wear unevenly. For
example, the inside edges may wear more quickly than the outside edges
or the fronts more quickly than the tails.
Of course, the real test is in the skating, and you'll gradually learn
the clues that point to a dull blade. You instructor can also help,
and if the skate sharpener is competent, he or she will do the minimum
necessary to your skates, perhaps just a quick pass with a hand stone
to knock off a nick.
Remember that every time you get your blades sharpened you shorten the
life of the blades and there's a bit of a re-adjustment for you to get
used to the new feel. It's almost always "better", but you get used to
when the blade will slide and when it will grab over the period as it
gets dull, and you may be caught off guard by the new behavior.