Have to be honest, that really is a how long is a piece of string question. Effectively your already at the bottom rung of the ladder in terms of no claims bonus, and generally you will lose 2 years no claims on an accident, but then expect a slight increase (which really can be anything depending on the company)
Ins. Premiums have already gone up by 30% in the last year, so it wouldn't take effect until your renewal, so it could be as significant as 50% (including the 30%), but it is purely a guess.
I haven't had an accident, and my premium went from £47 a month to £70 a month, and I have 9 years, I'm now paying £110 a month on an X5.