There are many variables as to how quickly the stored charge will decay. Our standard operating practice for 132kV paper/oil/gas cables was to isolate them from supply at both ends, wait 15 minutes, then apply the portable earth (or ground depending which country you read this in). This worked well because the insulation resistance in the paper cables did allow a good amount of discharge of the stored energy, down to a level where it was more than safe for the operator to apply the earth (ground). Plus at the time we mainly used magnetic voltage transformers for protection. This 15 minutes was calculated many years ago, and served well.
We came unstuck when we started to use XLPE cables, and also capacitive voltage transformers. Here you are automatically increasing the IR of the cable, plus you have eliminated the resistive discharge path through the primary winding of a magnetic VT and replaced it with the high IR of a capacitive VT. This showed itself when the operator isolated the cable, waited the standard 15 minutes, then proceeded to apply the portable earth, which drew an arc longer than he cared for whilst standing below it.
We did some rough calculations at the time, and found for the same stored charge, to discharge it to the same level that we believed would be safe to apply an earth to, the time rather than being 15 minutes (as for the paper cable) now became 14 days!!! We now tend to use a magnetic VT at one end of the cable now to allow the discharge to occur in a reasonable period, so we have eliminated the issue of arc-flash PPE requirements.
The calcs are pretty easy, you assume the maximum stored energy on the cable, then use the IR measurements from the cable (or at least the manufacturers IR levels, which will be higher then what you have in the ground anyhow) and the IR values of any VTs and CTs at either end. This gives a time constant from which you can tell how quickly the cable will lose the stored energy.
And as the above has said, don't assume it to be discharged until you can see that it is - earth or ground connected