How much +xp is worth keeping
Sockets are available at level 15, at that time I always end up getting a helm and socketing it for + % experience.
What I always ended up doing when leveling, like my level 40 Hardcore Monk, is every 5-10 levels I will check the auction house to see if there is better EXP gear available. I would always try to find EXP rings and amulets that still had at least one good stat other than EXP. The best form of EXP is having the best weapon available (what's the point of having a +10 EXP with a 15 DPS weapon when you can have a 45 DPS weapon and kill things 300% faster). If you check every 5 to 10 levels you should notice a good jump in the + EXP for each item and they should be relatively cheap.
Part of it also matters on how you level. Are you way ahead of the game for your level and fighting creatures that are +5 your level, and what you kill gives more experience and takes you a little longer to kill each mob, or are you a little behind for your level (as I am since I am Hardcore) where the + EXP means more both because I kill things faster and they give me less experience. Obviously, the + EXP equipment is worth keeping if you are a little behind.
In short: With my level 40 Hardcore Monk: I am still holding on to some of my EXP gear, but I always find the best one-handed weapon and shield. My + exp rings, amulet and other gear typically have one good important stat that helps (Vitality or Dexterity or Resistance). Every little chunk helps, if you kill a monster that is a little below your level and only receive 50 exp for it, the +52 exp as you mentioned makes a MAJOR difference (over 100% more experience). Even if you got 300 EXP per kill that will make it 352 EXP (17% increase) which in theory means you will level 17% faster. :)
As with Magic Find and Gold Find, the answer is "as much as you can wear without harming your ability to effectively clear content."
More +XP(/GF/MF) is never a bad thing. The returns are linear and there are no 'caps' that make some magic amount of it better than more or less. You either want +XP, or you don't. If you do, It will always benefit you more than overkill stats. But the instant you start having difficulty, it becomes a liability. Where you draw that line is up to you.