i.e. if I bought some now how long would be an ok to leave it unopened before it would start to go off?

2 months, 6 months, 12 months, 18......

Just we've got a good deal on the spectrum atm so wondered if it would be ok me buying some now with the offer and leaving it till say early next year.

Cheers :)

12 month shelf life unopened IIRC.

After opening you can keep it for about 6 months.

Unopened it is 1 year

Two gallons coming my way then :D