how long till i replace.........

[aquaman2]

what is the typical lifespan of these things?

MH bulbs (mine are about 9 months old) - is the 10k yellowing an indication its time to change.

RO/DI- (the whole system is bout 6 months old) - its 2 stage DI and the resin in the first 1/3 of the first stage is brown.

what else typically gets replaced?

 

[AZDesertRat]

MH bulbs generally last a year, but it also depends on how long your photo period is.
RO/DI should have all the prefilters and carbon filters replaced every 6 months. DI life varies and the only reliable way to tell is with a TDS or conductivity meter. Color changing resin is not very accurate. I change my resin when my TDS stays at about 3 or so.