Run JAGS and regularly save output to files.
This has the following advantages:
In the event of a power outage or other unforeseen system crash (eg, Windows 10 update), you still have most of the results so far.
You can get interim results without stopping the main process: start a new instance of R and read in the files generated so far. Abort the run if already good enough.
No need to exactly calculate the time needed for a run: make sure you ask for enough iterations, then abort when time's up.
Reduce memory needed in R (and never again get a "cannot allocate..." error after a long JAGS run): save as many iterations and parameters as you like to disk, then load only a subset of parameters or thin the chains before loading into R.
For more information see here.