If you place market order, then it will happen immediately provided there are buyers or sellers. In market order you are telling to sell/buy at best available price quoted by sellers or buyers. so in case of buy market order, you will get lowest price that is quoted by sellers and in case of sell market order, you will get highest price quoted by buyers.
But if you are to place limit order, then the time required to fill your order depend on how the price moves and availabilty of other buyers or sellers. What happens in limit order is your order is kept in queue, so that you can sell/buy to other people who place market orders. Some times your order may not even be filled, if the price moves against your direction
If someone agrees on your price quoted, your order will be filled immediately. (within a split second)
If no one agrees for your price, you need to wait until someone agrees (your order status will be shown as pending)
If someone agrees on your price but only for a limited quantity, then those many shares will get filled and remaining shares will be put to wait.
If many people have quoted the same price,you will be put in the queue as per your time of order placement. The person who placed orders before you, will get preference.
In summary, market order get filled instantly.
Limit order wait for your price to reach or your turn to show up if you are in the queue for the same price.