It can only be cast on areas that your line of sight allows.
This is a channeling spell; the caster is vulnerable to attack while they performs the spell.
Sadly, no normal mana regeneration is done while channeling far sight.
Casting another Far Sight while channeling a Far Sight will let you "jump" your vision further and further ahead. With practice, you can view anywhere on your continent with this technique. To do this, simply use Far Sight. Then, while channeling the spell, click your Far Sight spell, and move the camera further. You can move around corners, and far ahead of your normal line of sight.