How long will it take to get 99 woodcutting from 68 chopping ivy

With a rune hatchet = 208 hours or 8 and 2/3 days. (60,000 XP/hour)
With a dragon hatchet = 177.5 hours or 7 and 4/10 days. (70,000 XP/hour)

These are estimates while AFk'ing

Those estimates are if you played non-stop.