Just bought my own RODI unit for this exact reason, lol.Hahaha that is pretty funny to ask. This varies dramatically. One place I know of never changes any of their filters and were selling people tap water (this place was horrible). Many stores don't change the DI resin frequently. The place I worked would remove the DI resin if it got used up so it didn't dump things back in, but also did an ICP test to know what was being removed by the DI resin if they had to take it out (basically manganese, a little zinc, silica, and that's about it as far as I remember). They would only not replace it if there was no DI in stock.
People waste so much money and time buying store water. It's not bad if you have a small tank, but RODI units are much more cost effective in the long run. I have seen people come in buying 75 gallons of water and hauling massive amount of buckets at a time.