Araelus's post has the right numbers. Naturally, things that can't be merited a full 8 times (only HP/MP, magic skills, and offensive combat skills can) will be at full effectiveness earlier than level 65 (crit+ is at full effectiveness starting at level 40 e.g.).
Only three things to add:
1. You can only upgrade a given combat or magic skill if you have a 75 job that naturally has that skill. For example, if your first job to 75 is COR, then you can use that to merit dagger, sword, marksmanship, throwing, evasion, or parrying, but nothing else. If you wanted to pre-merit great sword for DRK, you'd need DRK, PLD, or WAR at 75 to do so (and having 75 DRK would defeat the purpose of pre-meriting the great sword for DRK...). By that token, you *can't* pre-merit the following because only the relevant job has the skill in the first place:
Katana or Ninjutsu for NIN
Blue Magic for BLU
Summoning Magic for SMN
Singing, Wind Instrument, or String Instrument for BRD
2. All the above stuff only applies to the five general categories (HP/MP, Attributes, Combat Skills, Magic Skills, Others). The remaining categories are job-specific, and they go by different rules. Fairly obviously, you must have that particular job at 75 in order to spend merits in its category. Furthermore, unlike the general category merits, job-specific merits do *not* apply at lower levels. They only apply when that job is your *main*, and is level 75 (so they don't work if you're level-capped and they don't work if you take after Cythe and like to be level 74 a lot).
EDIT: 3. (Thought of a third one.) Each category has both a combo total (total number of upgrades in that category, period) and an individual total (total number of upgrades you can put into any given thing within that category), as follows; if combo = individual, you can plunk all your upgrades into one single thing if you so choose:
HP/MP: Combo 8, individual 8.
Attributes: Combo 5, individual 5.
Combat Skills: Combo 20, individual 4 (defensive: evasion, parry, guard, shield) or 8 (offensive: everything else).
Magic Skills: Combo 16, individual 8.
Others: Combo 8, individual 4.
Job-specific (type 1): Combo 10, individual 5.
Job-specific (type 2): Combo 6, individual 3.
