This varies as to a number of parameters - battery types, usage habits, and charger type. What happens is that eventually the batteries get stratified. The sulfates in the electrolyte "stick" to the battery plates and coat them so that they just don't work as great as they used to. Equalizing batteries is basically shooting a higher voltage itno the batteries. This burns off the "coating" and returns those elements to the battery electrolyte, restoring it's efficiency.
You are likely to get stratification if your batteries use lots of water, which is generally caused by too high of a charging voltage in float mode and should be addressed. Also, if you short cycle them, never really run them up or down.
Some people find that they need to equalize every 6 months. On my 2004 Allegro Bus I equalized after 2 years and found that it really didn't need it. However, my batteries never needed water either. In 2 years I never had to add any.
Also, if you have AGM batteries, DO NOT equalize them. They do not require it and you can damage them.
Any time you equalize batteries you need to be sure that everything else in the coach is disconnected or else the higher voltage can damage your 12 volt equipment.
Mark & Leann Quasius
2016 Cornerstone 45A
2007 Allegro Bus 42QRP (Sold)
2012 Jeep Wrangler Unlimited - Rubicon