It usually takes about 2 hours to install a new tank, that's if the base is suitable for the new tank.
If a new base has to be constructed it could take 1-2 hours longer.
Good quality plastic oil tanks should last about 10-15 years but this depends largely on how well it was installed and that the tank has adequate support. Exposure to weather especially sunlight can be damaging to the tank also.
Yes, it is possible to have an oil tank in a garage firstly the garage must have adequate ventilation and must have fire rated lighting and switches and must have fireproof doors and ceiling.
An oil tank must be a minimum of 1.8 metres away from any part of a building or construction that isn't fire resistant, such as doors, windows, eves and cladding.
No it’s not safe to repair a plastic oil tank because it would only be a temporary fix and no telling of how long it would last for. It’s always best to install a new tank as the consequences of the old tank failing could end up being extremely costly.