How long do soul gem charges last?

I just crafted some Daedric war axes, with extra effect Frost and Shock. I enchanted each with a Grand Soul Gem. I decided I'd go try them out on some of the folks hanging around Whiterun. While the axes are indeed impressive, the charge ran out after killing a dozen or so people. Is this normal? Can anyone tell me how long charges last with soul gems? Thanks!

You can read the number of charges on the weapon when you look at them in your inventory. They will use a single charge -Per-Hit- on a target (so if it takes you 3-4 swings to kill your target there went 3-4 charges). Its always a trade off when enchanting your weapons like this, do you want to have more uses so you do not have to charge them up so often or do you want the enchantments to be more effective.

Hope this helps.

The following factors affect the soul gem charge of weapons:

• The size of the soul gem used in enchanting the weapon
• Your skill level in Enchanting
• Your skill level in the school of magic used by the enchant (e.g. Soul Trap -> Conjuration) (the more skill you have in Conjuration, the more times you can use Soul Trap before your weapon runs out of charge)
• A 'Fortify Enchantment/Alchemy/Restoration' potion that you may have taken (Note that this could be abused to create ridiculously strong 'Fortify Enchantment' potions. details here)

It is discussed in detail at this UESP wiki article here, complete with the formula on how it is computed: Skyrim: Enchanting Effects - Weapon Effects

Also:

You may have noticed the game engine bug that causes weapon enchantments to drain more charges after you save and reload the game. The price of enchanted weapons also jumps significantly higher.

These problems become worse as your enchanting skill increases. Once you've reached a high level and begin placing more than one enchantment on a weapon, the charges can drain extremely quickly after game reload. These side effects are all due to an error in how the game stores enchantment data, which causes it to incorrectly recalculate the costs associated with player-made enchantments after the game reloads.

This SKSE plugin fixes all these problems, so that you can now enjoy having an enchanted weapon that retains the same price and number of charges that were displayed at the enchanting table when you first enchanted it, no matter how many times you reload the game.

- from the Enchantment Reload Fix mod's description