Funky main fuel relay and/or RSR's can kill your engine.
I replaced both in a new to me 850 with KAE's and main fuel relay failed
in short order. I replaced that with a more expensive Straebel.
My, new to me 940, had a funky main fuel relay that would pretend to work
for months at a time and then quit.

These relays work with Volvo ECU black boxes and you can guess about why they act up.
If you have to solder it, then junk it, and test the circuit for excess current flow.

Just have a NEW main fuel relay and maybe an RSR in the glove box for your trip
and you should be happy as a clam at high tide.

Cheers, Bill

This is not an easy question to answer, because "it depends".

Typically, relay manufacturers will have datasheets that specify numbers of operations that will be in the hundredthousands to millions. But this is assuming you stay within minimum and maximum loads. The latter speaks for itself, but the former is less intuitive but a thing nevertheless.

High, capacitive or inductive loads may draw arcs when opening and closing and cause pitting of the contact surfaces leading to a decreased lifespan.

What I found on the main (aka fuel) relay on my LH 2.4 when it failed was cracked solder joints on the pcb inside the relay. A simple resoldering job resurrected the relay for probably another 25 years to come.
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1992 245 Polar B200F M47