After the leve timer resets, there are two conditions that grant a greater bonus when exchanging leves: 1) exchanging with a leve undertaken from a different city; 2) exchanging a leve given by a different 'issuing authority' (faction, i.e. Horn and the Hand). There's more info about this on Lodestone
. If you want to get the best bonus possible, you will have to do this carefully--so rather than the exchange being automatic, you have the option to get the most bang for your time, as it were.