The OSRS XP formula
Every skill in Old School RuneScape uses the same XP curve. The total XP for level L is: XP = floor( sum of floor(x + 300 x 2^(x/7)) for x = 1 to L-1, divided by 4 ). The curve is exponential: each level costs roughly 10% more than the last, and the requirement doubles about every 7 levels. Level 99 needs exactly 13,034,431 XP.
The 92-is-half rule
Level 92 costs 6,517,253 XP, almost exactly half of the 13,034,431 XP for 99. That is why veterans say 92 is halfway to 99: the second half of the grind is levels 92 through 99, just 7 levels but as much XP as the first 91 combined.
Worked example
You have 6,517,253 XP (fresh level 92) and want 99 at 60,000 XP per hour. XP remaining = 13,034,431 - 6,517,253 = 6,517,178 XP. Levels remaining = 7. Hours left = 6,517,178 / 60,000 = 108.6 hours. Progress to target: 0.0%, the whole second half still ahead of you.
Plan the rest of the grind
Once you know your hours, sanity-check whether the grind pays for itself: the OSRS GP per hour calculator estimates profit from your drop table, and the OSRS drop rate calculator tells you your odds of landing a rare drop along the way.