So Iv got a nice variety of mandarin from the supermarket and am growing it from seed. Iv heard fruiting can take yeeeeears of grown this way. Iv seen a fruiting bitter orange for sale online for £25, if I buy this and graft on the mandarin once it's big enough to graft will this mean the mandarin should fruit within a couple of years once the graft is established ?.
Update: Taking a bud or a graft from the mandarin seedling will result in a clone of the motherplant, therefore it will have the same genetic maturity and won't fruit earlier. For having your own mandarin fruits as soon as possible, take a bud or a graft from an already fruiting mandarin tree, thus making a clone of it, and graft it on existing rootstock.
Obsolete answer: Yes, the graft will produce fruit in a few years, but it will produce bitter orange, not mandarin fruits.