As of Sol 1802, a few days ago, Curiosity was at -4206 m MOLA. It landed at -4501 m MOLA, so the net climb so far is 295 m. Of course, it's gone up and down over that time, so the total amount of climbing is more.
The Mars Global Surveyor spacecraft (1997-2006) carried an instrument called MOLA (1997-2001), the Mars Orbiter Laser Altimeter. It generated what is still today the most detailed map of Mars' elevation. Along with the radio science from that orbiter and two others, we have a correlated gravity map to high order.
The MOLA science team arbitrarily but sensibly defined the zero altitude level ("sea level") for Mars using the MOLA data. To first order, it's the average radius at the equator, extrapolating an equipotential surface (taking into account both the high-order gravity field and the rotation of the planet) from the equator to the rest of the planet. More specifically, from the paper "Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars":
Zero elevation on Mars from MOLA is defined as the equipotential
surface (gravitational plus rotational) whose average value at the
equator is equal to the mean radius as determined by MOLA (cf. Table
4). The planetary radius and a gravity model derived from MGS Doppler
tracking data [Lemoine et al., this issue] with the IAU91 coordinate
system parameters for Mars [Davies et al., 1992a] collectively
provided the geopotential of Mars' mean equatorial radius. This
equipotential surface was then extended to all latitudes as the
zero-level reference for topography.
Prior to MOLA, a pressure reference was used. The average surface pressure of Mars is about 6.1 mbar, which happens to be close to the triple point of water. It was then almost poetic to use that as "sea level" for Mars, which we did. However there were huge errors in that approach, which was fine when we didn't know the elevation of anything very well anyway (except for the two Viking landing sites). Once we had MOLA data, a pressure reference was not usable.
The MOLA reference ended up being about 1.6 km higher than the 6.1 mbar reference. I remember a time when we had to keep asking if that altitude was a 6.1 mbar altitude or a MOLA altitude, and having to convert between the two.
As for how far it can or will go, no one knows. I would have never guessed that Opportunity ever could or would have driven 45 km, which it has. The only limit then would be the tippy top of Mount Sharp.