Any time you send troops, or your Hero, out from your Stronghold you activate a March.
The higher tier troops, the longer the march time. Mixed tier marches take on the longest march time of all troops sent.
March time is also affected by marching thru Wonder. This is particularly helpful when setting fake rallies, as it increases the march time for troops before they reach their destination.
Note: When attacking it is always best to send maximum troops possible, so that you inflict as much damage as possible with the least losses. However, when gathering for example, there are times that you want to limit the number of troops you send in a particular march. It is generally best, unless you have a large troop count, to send only the number of troops needed to clear the tile you are gathering from. There are exceptions to this rule however, such as when using a Resource shield, where you will want to send more troops than normally required.