The single timing chain on the earlier 3.5 E28 M5s (last ones had a double chain) and the 4 cylinder, single chain M3 E30 was generally replaced at around 100,000 miles costing around £1500 to £2500 depending on where and who.
The last of the 3.5 E28s and all 3.6 and 3.8 E34 M5s use a double chain. The consensus of opinion says change at 200,000 miles.
If the car has done 200k miles, then the chances are that the sprockets are worn, so these need doing too, plus the tensioner assembly *must* be done. Often the oil pump is checked or replaced as it is in the sump and driven by the chain too. A good mod is to use the E36 tensioner. Look for a thread on this, as it is said to be a superior unit.
It may be worth mentioning that, to my knowledge, no-one here on this board has ever suffered chain failure, nor personally heard of such a failure. There are a few cars here with starship mileage too.
Downside is that if it does go, then pop; end of M5. The the rebuild is more that the car would be worth in most cases if the vehicle has high mileage.
It is probable worth changing the tensioner at over 100,000 to 150,000 miles as this is the rumour-mills most often quoted point of failure. This will not cost much but must be done with care as it is a nightmare if done wrong or anything dropped into the engine!
Ditto Ivans comments
I have fitted the E36 chain tensioner assembly two weeks ago at 156k
This is the second tensioner fitted in my tenure of ownership.
It is supposed to reduce chain lash considerably which can accelerate wear on the drive faces of the sprockets leading to a chain potentially jumping a tooth or removing itself completely.
I fitted a new chain & sprockets @ 120 k when a rebuild was undertaken.
More as a precaution rather than a necessity as I had the parts for the cost of a good drink..!
The original chain was in excellent shape visually but the camshaft sprockets were showing considerable wear.
I have never heard of a duplex chain assembly failing on an M5 either...