My friend and I (okay... my friend) came up with the formula needed to calculate the exact amount of XP a weapon requires to reach a certain level. I can't believe it wasn't in the official guide. My friend never played Final Fantasy XIII. He studies in computer engineering, so he ought to know his math. Here's the formula:


TotalXP = baseInc*nbLevel + inc*(nbLevel - 1)*nbLevel / 2


baseInc being the number of XP required at base level (level 1)
nbLevel being the number of levels you want the item to gain (if you want to reach level 15, the value is 14 (15-1))
inc being the increment, that is, the additional number of XP which is added after each level gain

Example:

Weapon A starts at level 1. It requires 150XP to reach level 2. Once at level 2, it requires 175XP to reach level 3. At level 3, it requires 200XP to reach level 4. Its increment therefore is 25. Let's say you want it to reach level 21. The weapon would then gain 20 levels (as it already starts at level 1). Using the above formula, we'd get:
150*20 + 25*(20-1)*20/2
150*20 + 25*(19)*10
150*20 + 250*19
3000 + 4750
7750 XP required to reach level 21

I just estimate around how much levels I am away from the max, then multiply it by how much expereince i need for the current level. Since the amount will increase every level, I add in an extra Ultracompact REactor or two and boom I got a Star weapon.

The easiest way is just consult a chart, people have made quite a few charts already for every weapon and accessory and their respective requirements. 
 
This chart is weapons only, but accessories are pretty easy to figure out.