As boxtops says – they will normally take 2 years off.
With full no claims at 65%, loosing two years will typically take this down to 55% - providing no further claims are made the following year, the no claims discount would be 60% before finally returning to 65% the year after that.
So putting some money on those figures; suppose you have a base premium of £1,000. At 65% no claims your insurance will cost £350. When this is reduced to 55% no claims, the insurance cost will increase to £450.