Frustrated Conundrum

If I cast [frustration] and then cast [arcane conundrum] on the same foe, does the casting time penalty stack? i.e. 1 second cast takes 4 seconds.

nope
12chars

It will stack, but to an extent:

Activation time penalty will not stack to exceed +150%.

This according to the wiki:
http://guildwars.wikia.com/wiki/Casting_time

Also, it's not unusual to see mesmers in RA cast migraine and frustration on an opponent caster, maxing out the 150%.

I don't see where you came up with 1 sec spell would take 4 seconds. With frustration and arcane conundrum, it would be 150%, so 2.5 seconds if I'm not mistaken.