I don't think it depletes from just sitting there because the resin already removed what it removes. (Still not sure what exactly is happening in there). I make water every week and only see the color changing while water is flowing.
I had mine sitting for 2 weeks after I changed it waiting for a new membrane and it was still completely dark blue. There was about a half inch of water at the bottom of the housing

 

Agreed, with @latchdan, same thing for me

 

I've had DI resin last over a year then the next one only last a few months. It all depends on how often it is used. Simply change it out when all the color has turned.

 

Yes how much water is run through it and compounded by the TDS of the incoming water to the DI resin.

 

Agreed, and naturally occurring CO2 in the tap will burn thru DI resin faster as well. I make about 50 gallons RO/DI about every two weeks with high CO2 at the tap(well water), incoming TDS 10-20 depending on time of year. The anion resin last about 18 months, cation last forever, about 3 years now on my water.

Cheers

 

Mooner's comments are accurate - CO2 in the tap will burn through DI resin at a much faster rate than normal. If you're only getting 2 to 3 months at 20 gallons a week, that's less than 300 gallons of total production, whereas I would normally expect 1500 to 2000 gallons if the incoming TDS is only 10-20.

 

Without reading any of the comments, I'll add mine in.

There's really no time frame for how long DI resin lasts. When I had reef tanks and made RODI water I'd just carefully monitor what my output TDS was. If it was anything other than 0 I'd replace it because even 1 TDS means it's depleted. Depending on how many water changes I was doing, or how much evaporation I was needing to replace it could last 6 months, or sometimes longer. I'd also look at the color....if it was anything other than blue, it's getting close to or is already exhausted.

When I replaced it I'd usually swap out the rest of the cartridges. It was probably wasteful, but if I'm replacing one thing, I might as well replace the rest.

Best practice is have the materials on hand to replace everything and once you do, just reorder everything on the same day. The benefit is that you'll never not have what you need on hand.

ETA: forgot the actual response to your specific question. It's hard to say if having DI resin just sitting in water inside the housing for a long period will have an effect. The only real way to know is to measure the output TDS. Turn it on and let it run for a while, then test what's coming out. If it's above 0 TDS, it needs replacement. I've had DI resin sitting in the housing with water in it from when I moved a little over a year ago. Once I get the RO mounted, my plan is to just replace everything.