Soldato
Currently you can claim expenses for two years before its deemed a permanent place of work, so that could be a logical starting point. Even a year would be pretty reasonable as it wouldn't effect people working as maternity cover.
Personally I would be happy with Joksters suggestion. So long as you are contracted to a project that has and end date for your set of deliverables be it a bit part of a project or the whole thing and your employment is to be terminated upon completion that would be temporary.
In the real world I think a year is the best compromise.
I think that already happens. 4 month contract. Get to the end. Extended another 4 months, 3 years later you're still at the same company!