Whoever told you 3 to 5 years wasn't doing you any favors. If you take the 3 years you are doing it almost twice as often as 5 years. How much to do and how often to do it usually boils down to money and priorities. Other factors like envirommental conditions, how loaded the equipment is, how critical the equipment is and whether you have qualified personnel with the time play a big part in the decision too. There are so many opinions out there that I am not going to post anything free in an open forum to be debated back and forth. Why don't you contact a non-competetive business in your area that has similiar equipment and find out what they do. Visit their facility and note the condition of their equipment and find out if they have had any incidents. Some companies that have qualified personnel on staff will contract to do others medium voltage maintenance to help offset cost of training and workman's comp rates. Sorry for not giving you a quick short answer.