It depends.
If it is primarily for show, don’t worry about it.
If you actually use it, the following manufacturers have lifetime warranty
EOTech
Vortex
Sig Sauer
Leupold
Vector (only some top end kit)
Holosun
Of these, as far as I know, only Vortex, Sig and Vector offer a no quibble, transferable warranty, which makes the sights worth loads more second hand.