This depends on the type of riding and the amount of hours on the bike.
A two stroke running a little rich on the oil ratio will saturate the silencing media very quickly. On a big bore four frequent high speed riding will also shorten the life of the media. Also you should not allow water to enter the silencer outlet while washing your bike or ATV. Typically we would recommend every 15 to 20 hours on your machine to repack, but it differs for every level of rider.
Let your ears be the guide.