Currently, my five shamans are level 67.

My current rank of mana spring grants me 20 mana every 2 seconds. 10 mana per second, then, which is 50 mp5. Per shaman. Stacking, that's 250 mp5.
Water shield: 43 mp5
Gear: 52 mp5
250+43+52 = 345 mp5, or 69 mana per second.
My current Lightning Bolt rank costs me 189 mana to cast. It has a two-second cast time.
Two seconds of 69 mps = 138 mana. Over the casting time of my lightning bolt, I gain 138 mana, I lose 189 - total net loss of mana is 51 over two seconds.
25.5 mana lost per second.
I have 5937 mana total. This gives me 232.82 seconds (we'll round down to 232) of spamming lightning bolt, not counting latency of *any* sort (latency adds more to my favor, since it wind sup granting me spots where I'm not spamming LB like a clockwork program to-the-milisecond, those moments add up and give me back mana, so I can last longer before running OOM.)

232 /60 = 3.86 minutes of pure lightning bolt spam. This is without any other buffs of any sort in a latency-vacume. This is not counting any clearcasting talents of any sort. This is for a level 67 shaman in less-than-desirable gear (I actually stack stamina for world PVP against higher-level groups.)

Mana stream stacking is rediculously awesome for PVE.

Edit!
I just did the math for the major glyph to water shield. 4.2 minutes of LB spam. I'll get back to you guys later with the clearcasting talents, elemental mastery, and so forth added in. Even if nobody else is interested, I *like* this kinda stuff...