As synthetics have a more stable viscosity, from that point of view you can run them longer. As you can design them to be receptive of a multitude of additives without altering the stable viscosity they will last longer. But longer than what? Well, really only longer than a non synthetic oil in the same application. If you have a worn engine with bad rings allowing blowby, or incorrect carburation leading to fuel dillution for instance, even with zero oil leaks you may need to change it sooner rather than later.
I work in an industry where we do regular oil sampling as overstepping the mark can lead to six figure bills. That said, for us that isn't an option (unless someone works for Shell !) so passing on experience is the key.
Personally I think 2000 miles is way too conservative in terms of miles - but yearly is sensible especially if the car isn't used particularly regularly. With changes in temperature condensation forms in the crankcase even when in the garage and this ends up in the oil and "uses up" the additive (usually a calcium compound) that enables the oil to absorb water. When this is used up the excess water reduces the viscosity (not to mention corroding the bearings - thrust bearings are particularly susceptable to this in my experience, probably because of their location).
A leak is a leak - if it can be solved with a different type of oil then brilliant. But it shouldn't have any effect on oil change frequency. All you need to do is top it up, a little bit of new oil every week - total loss systems have their advantages
