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Totally depends on how it was installed, wood was cut and endgrain sealed, and how much maintenance has been done to the wood.
I've done several over the years, underslung sink (stainless is a lot easier to seal and manage than ceramic types), attached to underside of a perfectly routed out hole in the worktop. Sanded to almost glass level smoothness on the bottom where it will contact the sink, the top and the endgrain has to be ridiculously smooth. Then treat with whatever your chemical of choice and oil it, keep oiling it for around 6 months every few weeks without fail.
Last year I redid ours (had some deep cuts and a burn from a chemical spill), redid with tongue oil (not great, hard to work with, takes days to dry between coats and doesn't give much protection at all, but the colour is spot on), then smooth it all up again and coat in Osmo clear/ wax oil type protection. Been 6 months since I did that and maybe just about thinking about another coat of Osmo.
Sink wood area has a hard life, we don't have a dishwasher so area is in constant use, its a main sink and a small side sink with wood surface running between the two (in hindsight I might have just left it as one giant hole). It developed a crack in the first month of use (5 years+ ago) due to the oak being very thin here, handling and TBH I think I did stand on this part when doing something with the ceiling, filled and then been fine ever since. We have no staining, no mould anywhere never have, and no deterioration in any way.
Up until last years refresh, so between 6 months old and about 4 -5 years, if I gave it one quick wire wool job and a cotton swab of oil in a year, it would have been a busy year.
Where most folk go wrong is the relationship between the sink itself and the wood, too much gap, no enough gap, drip bead, no drip bead, use of silicon, not use of silicone, but mostly lack of surface finishing and treatment of the exposed end grain, every sink I've seen fitted in other houses this area has been rough and with many small defects which lets water gather there, or get inside the wood easily leading to at the least mould within 6-24 months of use.
BrickMan-
Thanks for the comprehensive reply.
My insurmountable problem with this top seems to be that it's been cut flush/slightly shy of the sink edge, no overhang or drip bead so silicon required, which seems to be a point at which the moisture is retained and the rot sets in. I also patched up a little bit with 2pac filler previously which seems not to have worked as it's just allowed water ingress around it and started to open up and root again.
I feel like the best solution is probably a new wood top cut with a bigger overhang and drip bead fitted without silicon, which is a compromise to the maintenance and finishing that'll bring, but otherwise means mismatching tops with the rest of the kitchen (where wood works just fine) or replacing the whole lot with stone/Corian etc. which I don't have the budget for. I guess the advantage would be that I could potentially buy and cut it ahead of time and spend a few weeks really going to town on the sanding and oiling.
I also hate the sink for it's tiny strainer trap but I don't think there's much that can be done about that without a larger remodel of the area.
Polygon
Are hardwood tops more resistant to rot? Probably gonna have to replace my worktop around my sink at some point as it's not in a good way, exacerbated by the fact it's cut to close to the Belfast sink with nothing to stop drops other than some manky silicon.