Fortunately I haven't had to do it yet, but by the look of it, about 4-6 hrs. Beisan have a DIY link on their website and I think their seal kits are far and away the best. If you ever need parts, try Schmiedmann in Denmark - I bought a head gasket vrs kit and set of cylinder head bolts for my E21 for AUD$150 including delivery via DHL and had it in 6 days. Locals quoted me $274!