When enchanting a sword to have Soul Trap, does the time mean anything? Seeing as you're going to be continually striking won't the timer reset? or does it count down from the first strike?
I know I've probably worded this properly but I hope you understand what I mean...

every strike counts as long as the item has a charge
in theory 1 second is enough if you manage to deliver the final blow
might want to go with a bit more in case some1 "steals" your kill (depending on how often that happens I guess)

More time is good because you won't have to hit your target as much and waste your enchanted weapons energy. I recommend putting soul trap on a dagger or a weapon you don't use often because

A. If it's your main weapon you will need to constantly recharge it =More money

B. You can use your main weapon to lower the enemies health and then use a soul trap weapon to finish them off.=Profit

You can also invest in conjuration and use bound swords. Once you get the right perks for them they can soul trap and don't have to be recharged.

Thanks for the tips guys; so just to clarify - I could use a soul trap enchantment of only 1 sec and as long as I delivered the killing blow with that, regardless of previous hits, I'd get the soul?

Enchant it with Soul Trap for 1 second and you will get the soul on the killing blow, no matter what hit it before. The lower the time on Soul trap, the more charges on the weapon, so recharging it really isn't a problem. I mean, if it has hundreds of charges, you don't really care, but I do like to pair it with Azura's Star and recharge it every time the star absorbs some puny soul.