I have 150 invested SPR (~200 with gear) on my Shadowmancer (Wiz2-Link1-Thaum3-Warlock-Shadow2) and usually if I don’t go crazy on spamming skills I can manage my SP enough that my rests are mainly for HP (Transpose + Bonfire or HP pot) rather than SP.
I don’t think ‘full’ SPR is worth it unless your build is going to focus on summons (Sorc variant)
Between the amount of stat you can get on high level unidentified gear and how often you’ll wait on Joint CD to burst down enemies (since you want the swell/shrink bonus, either more damage or more loot) around 200-300 SPR should be all you’ll need to not have to chug SP pots.
@Symphie : Remember that you can turn off the C2 attributes on thorn/conjur if you’re facing flying mobs/boss (the bottom portal for lv 300 dungeon for example) and save a lot of SP for basically no loss in damage.