I am making my own bow and I was wondering if there is any way to change the length of time needed to make a power attack for 1 specifc weapon. I ask because I want the player to be able to fire the bow very fast, but by only increasing the fire speed I still have to hold the attack button for a long time in order for the arrow to travel a long distance. So is there any way to change it?

That's probably what you're looking for: http://cs.elderscrolls.com/constwiki/index.php/FPowerAttackDelay

do you have any idea how I could use it in a script? It doesn't recognise the command.
Also do you know how to set it for just 1 weapon? That command will change all the weapons in oblivion since its a game setting, right?

FPowerAttackDelay is not a command, its a game setting, which can be changed by OBSE's SetNumericGameSetting.
Since the player can only have one equipped weapon at a time, you could add a script to your bow with begin onequip and begin onunequip blocks, which set set or reset the value.

But wouldn't the value also affect NPC's etc who the player would be fighting? I don't want them having this power attack increase aswell.

I have no experience with it, but the Wiki says:

That should only effect the player (from my knowledge of animations I doubt the 2nd part anyway)

Looking here http://cs.elderscrolls.com/constwiki/index.php/Category:Settings it says the whole game is revolved around these settings

The Wiki link I showed you does not look very revolving. Just try it. The following script should do the work (you need OBSE though, but thats no big deal either)

scn mybowfloat OldPADbegin onEquipset OldPAD to (GetNumericGameSetting FPowerAttackDelay)SetNumericGameSetting FPowerAttackDelay 0.1endbegin onUnEquipSetNumericGameSetting FPowerAttackDelay OldPADend