- We don't know of any firm upper limit to the distance the rings can transport.
- It would seem to be limited roughly from the surface of a planet to a close orbit of the same planet.
- The rings are based on similar tech to the Stargates themselves.
- I would assume the rings limitation is based on their capacitors or sensors...
So, there are no firm upper limits that we know of for pretty much anything in the Stargate universe.
Regarding the rings themselves, they are based on tech designed by the Ancients. They convert matter, transport it, and then put everything back together. While they are similar to Stargates they are not the same. I can think of three differences off the top of my head.
- The simple fact that they can transport matter both ways. We first saw this in the original film.
- Another big difference is that it is possible to intercept the transport with another set of rings, though I suppose technically it is possible to jump destinations, but doing so is much more complex.
- One final key difference is that you can also use one set of rings to transport without another set of rings to receive. In this the rings also have the ability to break through materials (i.e. structures) as can be seen in the film Continuum as well as a number of episodes in the series.
Given the obvious advantages of Ring Transportation over the Stargates there must be some flaw or short coming in them that caused the development of the Stargate technology over greater distances.
If Stargates were to have a flaw it would without a doubt be the limit in their capacitors. If a gate's capacitor attempts to store too much energy it will eventually cause the gate to explode, the key to both part one and two of the Redemption episodes. This I believe is also pretty much how Vala was able to destroy the first super gate in Beachhead.
I believe it would be logical to assume that a similar short coming is inherent to the Ring Transporter that keeps a limit to the distances it can travel. An notation in the SG Wiki reinforces my thoughts with mention of a buildup causing rings to explode during the Seth episode.
The other limitation may simply be the sensors in the rings themselves. If there is a "planet to orbit" limit range on the sensors themselves that could be why they are not used over greater distances. Sensors also came into play during the Lost City episodes when the team had to drill through the ice before the rings could be used to transport to anther set that were already within the outpost.
Whether the limit is one of these reasons or something else entirely I would assume with more development the ring tech could be improved and used over greater distances. This would likely never happen (even if the Alterans never left the Milky Way and) given the development of both non-ring transportation and the birth Stargate tech as well...