Nano tech is limited, like most other things, by the amount of energy it has access to and the material it can work with. Nanites can manipulate matter at the atomic scale so as long as they have the atoms to make something, they can do the thing you want them to do. This ranges from repairing DNA and cells to creating planets. If they have the material, energy, and instructions to do so they can do it.
As far as Time scale goes. That depends on distance to material and goal combined with travel speed and amount of nanites that there are. That's they could probably travel a few feet per second, lets say 1m/s. Assume all the material you need is at the edge of a 1m^3, have 1 nanite, and have a structure that will take up half the volume of the cube. Find how much material that is in atoms and then multiply by 2 because a nanite can carry one atom at a time and a round trip would take 2 seconds. This gives you how long that structure would take to be made with 1 nanite. For every extra nanite divide by that number of nanites.
Basically, think what you can do in minecraft with just picking up and moving blocks around. That's what you can do, on our level, with nanites.
There is however Femto- level tech which is the next level and likely will be called nanites just because people aren't too good with distinguishing things that are very similar to them, but on our scale their abilities could be very magical looking. Where nanites can manipulate atoms, femtites would be able to manipulate quarks.
Nanite manipulation could cause all types of chemical energy to be released, but once we get to Femtite manipulation we're dealing with nuclear energies and building atoms. Nanites you have to have the atoms to move around, Femtites you need the quarks from the atoms to make whatever atom you want. You couldn't change the mass, but you could change what it appears to us as.
Let's say you want to create an iron statue. Nanites would require that there be iron around to make the statue. If there is no iron around they can't make it and if there is iron in their search area but far away they have to travel to get it. Femtites on the other hand could just pluck quarks from the matter around them, create the particles/atoms needed on the spot, and provide you the statue. Want the statue to be gold? The Femtites just grab the particles needed and add them to the Iron atoms to make them gold atoms... Of course that's assuming that doing this doesn't blow things up, because doing this could cause nuclear explosions since a brute force way of doing this is how nuclear bombs work.
So again, when you think Nanites, think Minecraft more or less. When you think Femtites, think of what would appear to you as magic and alchemy, even though it is still essentially minecraft.