I think it should be instantanous when used in a gene/virus.
And I don't think it should be fireable with weapons as a settable thing.
Rather, I think that you should consider deleting genes a whole new weapon, different from the virus .delgene, or the .delgene defense.
How it would work is that, it would have to get past shell, slime, and even body (to a very minor extent.)
By minor, I mean that, for purposes of a defense, the target's body would be devided by their DNA length. So simple bots are practically immune to such assaults, while as uberbots that have an innate defense to viruses should be too 'bulky' to kill easily with it, but should generally always take damage.
How it would work is simple.
Simply storing -7 in .shoot will send off an attack, rather then an attempt to eat.
It's strength (which is, as all other shots, based on body, and should be improvable by expending some energy. In fact, that last one's a definate.) will be divided by a quarter of the target's Shell, then divided by a tenth of the target's Slime, and then divided again by the target's (Body / Gene Length).
This will build up until it equals 1, where, instead of a random gene being deleted, a random part of the DNA will be. (Such as an = in a condition, or something.)
Or it could just delete the gene, either way.
Anyway, that type of shot will also remove a bit of Shell, Slime, and Body. Not a lot, but not a little either. Perhaps equal to how much the shot's strength was divided by (except in the case of Body, which would probably decrease by (Body / 50) or (Body / 100), or something.)
Though all that's just an idea to keep .delgene a weapon, without having to use a virus.