Your shocks need to be serviced at least once every 6 months.
That does not mean they will fail if you don't. But it will save you
a lot of money in the long run if problems are fixed before they
damage other parts.
This does not mean a simple oil and nitrogen change. A shock
service done correctly will include replacement of known wear items
and seals. This also means it is being done by an expert on the particular
shock you have. Many of the wear parts are very inexpensive and a
good shock tech will charge you little for those parts and it will
save you a lot of money in the long run.