I am a lvl 66 atm so i dont have much exp with long boss battles. But ive have seen videos of battles that can last 8 mins and illidan can last 17 mins. My question is how can a mage or warlock's mana last so long? Sure theres mana pots but with a 2 min cooldown. My spells seem to drain my mana in 1 min.

kk well warlock has a spell called drain mana which can obtain a great amount of mana within seconds if used correcly.also there is another power called life tap which converts half your life into mana causing u to have quite a bit of mana for a while

Also mp5(mana per 5 seconds while casting) is a great help to all casters, i myself m a holy pally and the only way i stay in the game is with my mps. and its also about how u use ur spells. dont use your highest mana sells early off. an easy way to avoid loosing mana fastis to get the mana oil from enchanters that do 14 mp5 the spell power one is good but if ur mp5 is low use it. also look into talent points that give u mana back.

Other things regarding long fights, especially in raid groups:

1) Raids often rotate caster groups so that some casters are out of the five second rule, and therefore regenerating mana.  This is especially important with healers.  Some do this formally, some let the casters work this out.  I've seen it done both ways successfully.

2) Raid casters (especially healers) often have an insane amount of mana/mana regen.  It is not unusual to see raid healers with 450-550 mana regen (both spirit based and from MP5 gear) and over 12k mana.  It takes a lot to suck that dry.

3) Most casting classes have spells to regenerate their mana quickly while in combat.  Some classes have the ability to give mana back to the caster line. 

4) Raid casters are not full-time casting.  Those battles often require a great deal of moving around to get away from various traps, weapons, bombs, adds, or while the tank is moving the boss into x position (Quick!  Everyone behind a pillar!). Most of those long battles have such 'lulls'.  A few seconds out of the 5SR = a lot of mana. 

5) Spells are cast to give the most sustainable dps/hps per mana while keeping all casters sufficiently below the tank's aggro.  This often means using smaller, most mana efficient spells/heals to spread the threat out. 

At level 66 you are probably just beginning to see Mp5 and socketed gear (gems can give you Mp5 as well) but it is very common at lvl 70.  Both my raid healers have muliple trinkets to give them mana as well just because ooming is simply not an option.  If the tank goes down it better be because I died. :p

Hope that helps! Best of luck!

thx for the advice you guys!

Pots have 2 min CD, so if you use them at about 70% of your mana, the CD will be over at about 50%, so you take another 1,
mages also have evocation, so they get +60% mana with 8 min CD, and than the conjured stuff gives them mana as well with a 2min CD (not shared with potions), and ofc your gear with MP5 or mana regain when crit.
:)

Along with mp5, spirit and pally stuff, its always good to have at least on1 SP and one resto shammy in a raid too. Resto shammys have mana spring, and the coveted mana tide. An SP dpsing means health and mana to the party as well, always a good idea to have that. So with properly mixed and matched classes in a raid, you wont run out of mana.

lololol..
normally for the ilidan fight you wil group up warlocks / mages with either a shadow priest or a shaman. paladins cast vindicators strike on boss = you get 74 mana each time you make dmg on him, shadow priests use vampyric touch = you get 70-80 mana every 1.5 seconds. shamans can do mana spring totem and mana tide (cant remember exact amount of mana obtained here)
but beside this you can get food / potions to buff up your mp5 (mana per 5 seconds)all in all if you are in an imba grp (meaning Sp and shaman) you can obtain an MP5 at about 400-600 meaning you have a hard time going oom.. if you do so anyways go for a mana pot.. cheap but efficiant.

/nagasaki of magtheridon

Shammys mana tide totem is gold, and also they key is to drop the pot at like 60% so u get another one before the cd