I thought I'd sit down and think about this for a while - it's a slow day at work.
Spellsurge has a 15% proc rate, according to the tooltip. Some people say they think it's more but for the sake of simplicity, let's assume that it's 15%.
Flash of Light takes 1.5 seconds to cast so if you're spamming FoL, you're going to get a proc every 10 seconds. Add in the cooldown and you're getting the procs every 40 seconds.
You get 100 mana in the surge, so that's 100 mana every 40 seconds, so effectively putting spellsurge on your weapon adds 12.5 mana per 5 seconds while casting.
That's what you (and every caster in your group) gets.
If I've mis-calculated, feel free to correct me.