Have you ever wondered things like, "How long, Lord? When will You answer my prayer? When will I...?" and a variety of other questions. The context of the verse mentioned here is the summary answer Jesus gave to His disciples concerning their questions concerning His coming and the end of the age. (Take time to read the chapter).
It is not uncommon for us to wonder "when". We want to take God at HIs Word and so, we expect answers from Him. For example, God gave Abraham a promise that he would father a son; but time went by and there was no child. Abraham and Sarah determined to give God a hand. The plan was made that he should father a child by Hagar, thus fulfilling the promise. We all know how that turned out! But later, according to God's timing, a son was born, just as promised.
Delay does not necessarily mean denial. Reading 2Peter 1:1-11, we find that scoffers doubt the promise concerning the Lord's coming...but His seeming delay holds the promise that the reason He delays is so that many more may come to repentance. Prophecy requires a series of events to reach fulfillment. God has a reason for His seeming delays.
But, when He begins to bring to pass His promises, things tend to progress rather quickly! The phrase "it came to pass' occurs some 450 times in the Bible, indicating that eventually things happen. Perhaps this phrase occurs so often as a reminder for us to patiently wait on God's timing to bring about His plan and purposes in our lives. Eventually everything God has promised will 'come to pass'.
Everything can change in a moment when God works 'suddenly". He often works suddenly. It was a typical day when Moses was going about his daily shepherding duties that his attention was arrested by a burning bush. Joseph was a prisoner one minute and the next was summoned before Pharoah and became second ruler of Egypt (according to a promise made years before). It was suddenly that the angelic host appeared to lonely shepherds and announced Christ's long awaited birth. Saul was one day 'breathing out threatenings and slaughter against the disciples...and suddenly there shined round about him a light from heaven:' Acts 9:1,3 KJV
Time and time again, we read how God acted 'suddenly', but only after a long period of time.
Since He has shown us that He often works 'suddenly'; why do we grow discouraged when there is a delay in answered prayer? Delay does not necessarily mean denial. A quote by an unknown source reads "Faith shouldn't die if the answers come slowly, for the delay may be God's way of working His will in your life."