4-5 years max I'd give them or even less if the bikes have just been sitting unused.
I had a bike a few years back that I'd used very little over a 2 year period, got back into it and I could not remember it handling so shitty! I went through everything - fork oil, bearings etc etc and nothing improved - I was complaining to a dealer I know and he asked how old the tyres were - they had heaps of tread - I changed them and hey presto the bike was transformed.
Basically the rubber hardened from lack of use.
With motorcycle tyres being so critical to keeping you the right way up, my motto is if in doubt change them!