Thanks for all the replies, seem to remember back in the 90s that insurance companies wanted at least 120 minutes. I am only giving it as business continuity advice as a fire in an un-sprinklered part of a building could develop to such an extent that once it breaks into the sprinklered part it could overcome the sprinklers in that compartment.
As Phoenix point out tb206 gives a range of recommendations and we are all in the business of mitigating the effects of fire. But some of the recommendations in tb206 are unrealistic. Take as an example the recommendation for 4 hours separation (if I recall correctly- not looked at it for a couple of years) between sprinklered and unsprinklered parts of a warehouse. This is not easy to achieve in modern steel framed buildings.
But some insurers jump on it and try to persuade the client to implement it often with all sorts of veiled threats. The fact is that if you ask them how much will be saved on the premium, or whether the premises risk profile will be reduced the answer is always negative. The risk profile is determined by the overall property portfolio.
For this reason one of my clients used to construct to meet building regulations only usually to a 1 hour standard and the insurer grumbled but never refused cover or applied any special sanctions.
The water infrastructure for a sprinkler system is generally designed to provide cover for one hour.