It depends on the cause of renal failure
If it is acute kidney failure - dialysis
will be required till you kidneys regain about 15 percent of their functions on treating the cause of the renal failure.
If it is a permanent damage then dialysis or transplant remains the only option.
Some patients may have a chronic kidney disease
with partially functioning kidneys, who normally don't require dialysis, but their kidney function has temporarily worsened due to some other cause and may require short term dialysis.
There is no specific value at which dialysis is started but depends on variety of reports and patients overall condition ( some even require it at a creat of 3 while others may be fine even at 7 or 8)