What are radar calibration spheres
I couldn't find any specific information about the SKRL calibration spheres, but based on descriptions (passive, spherical satellites) and information about other radar calibration satellites (LCS) I'd agree with your guess that they're simply hollow metal targets.
Radar calibration requires known data to compare with the output of the radar to allow tuning of the radar such that the radar output matches the expected results. Calibration spheres are intended to have a known, constant Radar Cross Section (RCS) value when viewed from any angle, so they're most useful for calibrating RCS (though if the satellites can be tracked with a more precise tracking tool, like lasers, the same satellites could potentially be used to calibrate radar position data).
Fundamentally, the calibration satellites can be used by any radar that has the range to reach space that the satellites will fly over. Considering the inclination of the satellites is 82.42° only the poles won't get direct overflights, but with the ~600km altitude of the satellites should still be visible above the horizon at the poles. In terms of what radars would actually be capable of using satellites for calibration the radars of the Space Surveillance Network, though when you look at some of the radars that make up that network a lot of the "Contributing Sensors" are Ballistic Missile Early Warning System radars.
Between that note and the fact that RCS is used to characterize objects (if you confidently know how big an object is in terms of its radar reflection you can make a guess at what you're looking at) I'd guess that the primary reason for launching the calibration spheres is to calibrate Russian ballistic missile early warning radars (though if the expected RCS of the spheres was published anyone else could use them for calibration too) so they're best able to determine what they're tracking based on the RCS detected. I know basically nothing about Russian ground-based radars so I can't speculate on specific sensors or target precision.
Where they are
This page tags the calibration spheres as having NORAD identifiers 39490 and 39491. The current (July 2015) orbital parameters you asked for (follow the links for full TLE/kepler elements/state vector) are:
- SKRL 756 1: 82.42° inclination, 96.8 min period, 592.0 km - 615.1 km altitude
- SKRL 756 2: 82.42° inclination, 96.7 min period, 584.4 km - 614.6 km altitude
This NASA page claims that "Debris left in orbits below 370 miles (600 km) normally fall back to Earth within several years." though going back to a much earlier (December 31, 2013) element set for SKRL 756 B indicates an altitude of 592.0 - 622.5 km orbit. In a year and a half we've seen measurable but minor decay, but that will speed up as the perigee drops lower and encounters more atmosphere. I don't have the tools to really calculate a timeline for decay but I'd estimate it will be more than several years but probably not more than a few decades.
To answer the comment: "is aluminum or another material used?"
Again, I don't have details for SKRL, but I do have reference material for LCS (Lincoln Calibration Sphere):
...a precisely-machined and carefully-polished rigid aluminum sphere of unit square meter optical cross section. ... The sphere was fabricated from standard certified 0.125 inch 60-61 aluminum stock.
Also, the first hit on Google for "radar calibration sphere" is this company that's advertising/selling "Hollow Aluminum Calibration Spheres".