This sounds like there is a problem with 1) the material the crowns are made of and the skill of the laboratory making the crowns- some materials are very technique sensitive and if not managed properly in the laboratory setting failures can occur OR 2) the occlusion or the "bite" has not been properly made to reduce forces and prevent breakage. While some repair from time to time is normal, after only 2.5 year that much fractures seems to indicate a bigger problem.
Is it possible that
your husband has a grinding/bruxism/clenching habit? If so, the constant wear
of the porcelain crowns against each other may have caused them to break down
long before they should have. After having new crowns placed he may need to start
using a bite splint at night or during the day if the dentist suspects this a
problem. The crowns most likely should have lasted closer to 10 years, and
hopefully your dentist will work with you on reducing the fees on his end.
Repair/Replacing dental implant teeth/crowns
A full set of upper and lower implants is an investment that should last longer then 2.5 years. However there are a variety of factors that effect the longevity of implant crowns- most importantly the materials that were used by the lab. Some materials used for crowns (such as all-resin) are lower in cost, but are not as strong as other more expensive materials. Therefore it is important to evaluate different material options with your dentist before investing in dental implants.
Aside from the actual crown fabrication your husbands treatment of his teeth could be resulting in their breakage. For instance if he grinds his teeth at night, favors eating with one section of his mouth or tends to clench his teeth.
With proper care and routine dental check-ups dental implant crowns should last a lifetime.
How often do dental implant teeth/crowns have to be repaired/replaced?
That all depends on what was done, how it was done and what your husband is doing with it. From the fee you mentioned it looks like your husband had a pretty complex restoration - that definitely increases possibility of future breakage/repairs. By "how it was done" I mean from the way implants were placed to quality of lab work, to the way his bite was restored there are so many variables that are difficult to control. Finally, what he eats and how he chews as well as any night time grinding/clenching also determines how long restoration will last. He should be wearing a night guard to protect his investment.