Nick what do you mean "for how long", new windows increase a houses
value, save winter and summer in utilities from R value and less air
infiltration, and the UV reduction saves everything inside from damage.
Warranties of 15 years are normal, with the new sealants introduced in
the last few years condensation may not occur for 30 years. Payback
keeps getting less as utilities go up, I see no reason to keep old
glass, unless you can`t afford new glass or it is architectural.