No reason for the tensioner not to last the lifetime of the bike.
Regarding the chain, the bikes that I have rebuilt have such unreliable mileages that I haven't been able to fathom how long a chain lasts - sorry! But if your chain isn't noisy then leave well alone
Also, the chain is relatively cheap so I usually fit a new one on my builds just as a matter of course.
No reason for the tensioner not to last the lifetime of the bike.
Regarding the chain, the bikes that I have rebuilt have such unreliable mileages that I haven't been able to fathom how long a chain lasts - sorry! But if your chain isn't noisy then leave well alone
Also, the chain is relatively cheap so I usually fit a new one on my builds just as a matter of course.