This link gives some indications of when a reagent is compromised. I would say a reasonable rule is the following for reagents that have been opened and are stored in room temperature conditions away from sunlight. Others can make comments since some of this is subjective. Taylor generally recommends replacement after 1 year.
1-year (maybe 2, but check condition; sensitive to moisture and heat)
R-0870 DPD powder (N,N-Diethyl-p-phenylene-diamine sulfate, EDTA, phosphate buffer)
probably 2 years but most of these are sensitive to heat; FAS-DPD is sensitive to light as well
R-0871 FAS-DPD titrating reagent (ferrous ammonium sulfate)
R-0004 pH indicator (K-2006; phenol red organic dye and chlorine neutralizers)
R-0014 pH indicator (K-1000; TF-100; phenol red organic dye and chlorine neutralizers)
R-0600 OTO indicator (K-1000; TF-100; organic dye)
R-0008 TA indicator (organic dye)
R-0011L calcium indicator (organic dye)
R-0007 thiosulfate chlorine neutralizer (sodium thiosulfate)
R-0012 hardness reagent (EDTA) -- published shelf-life 18-48 months
R-0003 DPD reagent #3 (CC test; potassium iodide) -- sunlight plus oxygen oxidizes it
3 years or more
R-0005 acid demand (sulfuric acid)
R-0006 base demand (sodium carbonate)
R-0009 sulfuric acid titrating reagent (sulfuric acid)
R-0010 calcium buffer (sodium hydroxide high pH buffer)
R-0013 CYA reagent (melamine in a low pH buffer)