Yeah...kinda instantaneous really...
You know...the advances that have been made since the creation and subsequent use of the bombs on Hiroshima...have been quite dramatic and very clever.
You see...Atomic bombs, arent very efficient...they tend to blow most of teh reactive "Fuel" elements of themselves apart and hence are unuseable in teh reaction...what they have done now...is refined the the things quite nicely...they use a Fusion reaction initially...which provides fuel etc for a bigger Fission reaction...also, to try and contain the thing as long as possible...they create something called a Tamper, which, is also made of a fuel yielding material, which further increases the reaction...which all in all gives massivly greater yields (in kilo tonnes) than teh original bombs did those few years ago...typical warheads are actually capable of 5000 times the yeild of Little Boy.
Frightening, but fascinating...