TLDR: 1x lores after lvl 50 for 25 religion, 24 summoning. Don't count on 1615 to kill, but it does good damage.
Now for the long version:
Lores and casting in general is very much a level based question.
Most paladin builds will not get lores until after level 40. People generally start out 1x in spells to quickly get 1625 and 1635. Because we are expected to .5x spells and have costs in line with that, going 1x eats into the MTPs alot, leaving no points for lores. You have to either slow down in spells to free up the points or wait until stat gains give you more to work with.
Once MTPs are available, the lores trained in will be mostly determined by the overall build. The exact numbers vary, but in general zealot using paladins will focus on religion lore, more spell heavy paladins will focus on summoning, and more defensive or shield heavy paladins will focus on blessing lore. Everyone will get 25 ranks in religion lore (for kneeling effects) and most people will get 24 ranks in summoning for 1630 targets.
As mentioned before, MTPs are in short supply for paladins. Unless you have an uncompromisingly heavy mental build, you will probably not fit in more than 1x lores after level 60, capping with 40-50 total lore ranks. Your best bet is to put the first 25 in religion and then add other lores to hit whatever thresholds you find desirable. I like to aim for 25 religion, 24 summoning, 52 blessing at 1x (post cap) and 25 religion, 77 blessing, 100 summoning as final goals (very post cap) for a heavy spell/defensive build.
There are two competing factors when it comes to using spells for damage as a paladin. Both are sort of rooted in that "expected" .5x per level spells. On the plus side, creatures are balanced against that expected training. This means that a paladin taking the more common route of 1x spells is significantly over-trained in spells. Everyone rushes up to 1635 or 1640 by level 35 or 40. The game however is expecting us to .5x spells and put some of them in minor spirit. So at level 40 we have ~40 paladin spells when the game is balanced for you to have more like 10. This means your CS will let you hit just about anything with ease.
On the flip side, dumping all of those MTPs into spells means you have no room to heavily train in harness power. Couple that with our cheapest damage dealing spell being 15 mana and you end up with an attack that always hits but cannot be used many times per hunt. 1615 has a goodish kill rate on crittable creatures, but it is not a consistent one shot kill. It is obviously designed as more of a setup to be followed by melee attacks or as a group contributing spell and is most effective in those roles.
In the normal progression for a paladin, 1615 starts to be practical around level 40; when it helps against select more-difficult targets in the hunt. It will become a standard setup ability on all foes sometime between levels 65 and 100, depending on the build.
I have known a couple of paladins who used a completely 1615/30 based hunting strategy. This is an extremely uncommon build as 1615 only has a single chance to crit on the flare and is very mana intensive if you try and bleed things out. As a mainstay hunting spell it either requires a lot of post cap exp or some very specific strategy. Those strategies are either always hunt in a group (generally MA groups) where you only need the "tag" or hunt something that gives 1615 a higher than average chance to crit kill (generally fire against troll warcamps).