Dungeoneering experience can be bought from the Rewards trader in Daemonheim. It is bought at a rate of 1 token for every 1 experience point. Tokens are earned at the rate 1 token to every 10 experience points (rounded down). As a result, by simple calculation, a player gains 10% more of original experience if he/she uses all the tokens for this reward after a dungeon raid. Doing so is only recommended if the player has already purchased all other desired rewards or if they have no interest in them.
If no reward tokens are spent on items, you could get 99 Dungeoneering by spending all of your tokens on experience when you have 11,730,987 experience points (or level 97). Likewise, it is possible to achieve 120 Dungeoneering by spending all of your tokens when you roughly have 93,600,000 experience (level 118).