There is no single correct answer to this. Much depends on three things:
1. what a given organisation is describing as major or minor, because there are some significant differences between organisations. http://www.hartech.org/docs/service_prices_944_jan2011.pdf is a useful reference point.
2. what other work is known to have been done in the last three years, and known to have been done to a high standard.
3. what your intentions are for the car versus the shape it's in now.
Cars are intended by their manufacturers to wear out and be scrapped in a finite time, something between 10 and 20 years depending on the manufacturer's philosophy. Once you are outside that design life, which every 944 now is, then the question of routine servicing versus rolling restoration becomes very blurred, and the individual needs of a specific example of the vehicle start to dominate any "standard" view.
If you want a car to continue to operate in a relatively fault-free way over several decades, rather than the 10 years or so for which the original manufacturer troubles to concern itself, it needs to be inspected and maintained to a far more intrusive level than any factory service schedule., simply because the factory service schedule was never intended to actively prevent and reverse age-related deterioration over a 20 or 30 year period. For example, it is not a routine service item on OPC schedule to lift the cylinder head, change the gasket, de-coke it and regrind the valves. But if you were sitting down in 1980 with the designers and saying "Should I anticipate having to do this in 20 years' time." they would have said "Of course you should".
Similarly, I don't think it's part of any manufacturer's 'book' service schedule to replace all the hoses. But obviously they are consumable items that suffer from age, so a service schedule designed from the outset for a 30 or 40 year life would include hose replacement. Things like timing belt life, the best practice now is very different from what was in the factory schedule when the cars were new on the market.
In other cases we have found empirically what the lives are of components. If I were buying another 944 now with a view to keeping it long term I would as a matter of routine budget for things like a new DME relay, fuel pump, FPR and AFM overhaul, because I know that these are things which either need doing or will need doing at some point in the next decade, so I'd rather do them now before they go wrong than wait for them to go wrong at an inconvenient time. if it were a Turbo I'd replace the drive shafts and CV joints, same reason. But if I bought it from someone on here who I know had done all that in the last three to five years I wouldn't bother.
That doesn't mean you have to completely dismantle the car every year. If you had a very intrusive, heavy-duty inspection and overhaul the year before, like a Hartech Gold Major service carried out on the basis of "please do whatever is helpful to extend the life of this car indefinitely, replace anything that is worth replacing while you have the dismantling done, and while you're at it do a complete suspension rebuild, refurb the calipers, belts, pulleys, oil seals, do a full top overhaul etc etc", then if you come back a year and 5000 miles later later and it's running like a top, then it won't need the same again. It might well need a pretty thorough inspection, but little actual repair work.
Really you need to work with your maintenance provider and discuss with them exactly what needs doing when, and what your objectives are. Do you genuinely intend to keep it running as perfectly as possible in an open-ended way, so that it is instantly usable and has much the same everyday reliability as a new car, and will look and drive and feel much the same in 2025 as it does today? If so, you need to get your maintenance organisation understand that and act accordingly. Or do you just want a stamped up service book with the factory schedule items done, and accept that things which are not included in that regime will occasionally go wrong and need repair between annual services?
Most organisations will assume that you intend to run it for a couple of years, keep it reasonable and then maybe move it on, in which case you probably won't want to do a lot of things which head off trouble more than a couple of years down the road.
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