Yes, time between buckets affects it much. Every component in your reactor can store 10000 heat itself. It means if you have 20 HD's in your reactor, its overall heat storage increases by 20 * 10 000 = 200 000 units. But, by adding lava buckets, heat is added directly to reactor hull, not to components. After that, you have to wait few ticks until that heat is divided evenly to every component in reactor, which has connection with reactor hull. Every HD is able to exchange up to 25 heat every tick, which is 500 heat every second. After adding bucket of lava, your HDs will immediately start to spred the heat from hull evenly to all components touching them, but that takes some time. If you add 5 buckets at once, reactor hull goes critical and it explodes immediately, since HDs are limited to 25 heat per tick (each). Better description of mechanics can be found on wiki or in reactor tutorial.
Aaah, I see. So that time is dependent on how long it'll take for the heat to get out of hull and into the components. And after some soul-searching, I've realized that the heat will be dispersed throughout the HDs until they are equal with the hull, and as long as it's neutral, the hull will keep that level of equal heat in everything, maintaining the heated hull environment, so that's why I need so many buckets of lava. Awesome, now I can start designing breeders with comprehension. Thanks.
Try to avoid using that much Heat Dispensers in your next one.