Ok so I'm thinking about making a custom Drain Health Spell but I'm not sure how long I should have the spell last I was originally thinking 100 points for 1 second but I don't know if thats enough time for the spell to work.

You understand that Drain is different than Damage, right? If the enemy has more than 100 health, after 1 second, all that health will return.

Yeah I know it'll come back I'm just not sure if I should have it at 1 second or more then 1 second

If the enemy has less than 100 health and you cast that spell, he will die.* If the enemy has more than 100 health, the spell is pretty useless, because the damage will not last long enough for you to do anything else.

EDIT: *On normal Difficulty, and you're not wearing armor.

That will cost you a butt-load of magicka to cast though, why not drain health 20 pts for 5 seconds?

Drain Health 100 for 1 sec is normal, and very cheap to cast. Couple it with spell chaining plus Weakness to Magicka... dead enemy very quickly. AoE is even more ridiculous, and adding Calm or Chameleon is... well, you get the idea, I'm sure.

Because the enemy will get all his health back after 5 seconds, if he had less than 20 health during then fight he will die else he would take no damage. Drain health temporary removes health and return it then the spells times out. this is why drain health 100 for 1 second is so powerful. Drain health 20 for 5 seconds will only have an effect if you cast spell then does other attacks and bring enemy health down to 20.

You might mix it with absorb health who takes health from enemy and gives to you. Absorb skill and attributes works like drain. absorb strength 20 in 5 seconds will do a drain strength 20 on enemy and fortify strength 20 on you. However absorb health, magic or fatigue works like damage health, magic or fatigue on enemy and restore health, magic or fatigue on you. If you are at full health the healing effect has no value but enemy does normal damage. Absorb health don't work on bows but is extremely powerful on melee weapons as it keep your health topped.

I do understand drain health. But it sounded like he was talking about the spell being the ONLY spell he would cast. Which sounded crazy to me. So extend the duration for that extra time to cast other spells.

The only attributes worth using a Drain spell for are the ones that help the enemy fight. Drain his speed, drain his strength etc. and you get hit less often, less hard. It's a defensive spell, because it reduces the damage to the player. If you want an offensive spell, use Damage instead.