Mine have been on since last winter. Didn't use the car much over summer and we have had a wet year anyway.
Tyre companies and VOSA don't say you shouldn't do this in the UK but they do state you will have worse wear rates.
I hope to put Michelin Primacy or Energy on the car in March, and take off the winters, if I can afford it I will buy another set of wheels, 17", other wise I will remove and then refit in October/November.
I think I have been lucky in that I rarely go on long journeys these days, as the extra heat from a long hot motorway run would probably really increase wear.
I do a school run in the AM, 2 miles normal road, 14miles Motorway, then 15 miles on normal roads with some slow speed urban driving near the schools.
Twice a day explains the need for an economical diesel car! Especially at £7 odd a gallon.