Depends on the content you're doing. Killed Zon'ozz 25 heroic as Holy tonight, and I was struggling for mana towards the end, with 3100 spirit passive + normal mode Heart of Unliving. I was playing quite inefficiently, though. For normal modes, you could easily drop below 2500 spirit with Heart and be fine.
I'm running 2850 + Tsunami + HoU, so about 4150 with them stacked. I think I'll end up going Spirit in every slot unreforged + HoU and the trinket that procs off crit heals (apprently it's pretty damn good, low ICD) and I should be at a nice spot.