Back to the OP. The information beyond the Ilford data sheet posted at the beginning of this discussion) can be confusing and sound ambiguous to someone with limited knowledge and experience of the photographic process. First, there was a statement by the OP that makes it clear he does not understand how f-stops and time relate. If you close down the lens aperture to the next smallest (higher number) stop, the time doubles. 2 stops and it doubles again--quadrupled from the original time, and so on. Secondly what size prints are you making? 3-5 second exposures at f8 seems too short to allow you to dodge and burn your prints (as you get more confident and gain darkroom skills). You might need a lower-wattage bulb or an ND filter in the filter drawer if your enlarger has one. find 6-7 seconds a minimum, comfortable time, but sometimes go up to 20 seconds. Also, larger prints require more time. I generally figure about 2x for every standard size up. For example, if the correct exposure for an 8x10 print is 10 seconds at f8, it will be approximately 20 seconds for the same negative enlarged to 11x14. Just a ballpark figure that usually has to be fine-tuned.
Back to the OP's original question about developing time, here's an easy experiment. I will assume you have been able to make a print you like. Using that exposure, develop the print for 2 minutes. Now, make another--same exposure time--and develop for 4 minutes. Then a third, developed for 6 minutes. After fixing, washing and drying, compare the print side by side in even light (not bright sunlight!). Is there a difference in the blacks between them? Is there a difference in the whites? If there is no difference, you're good to develop all your prints for the rest of your life at 2-6 minutes and never have to think about it again. If the 4-minute blacks look a little deeper, then 4-minutes is your time. If the in 46-minute print, the whites seem a tad dingy, you know that was too long in the developer and stay with a shorter time.