"Considerable time" refers to a significant or substantial amount of time. It implies that a certain period, which could be long and undecided, is required to complete a task, reach a goal, or make a decision. Full definition
When time allowed, she also spent considerable time in the Tea Room. (windsorcourthotel.com)
They go thru behavioral training, agility training (if applicable) and generally are worked with for considerable time to assess the best home. (onegreenplanet.org)
That said, it's also important to remember that even with those big gains, it can still take considerable time for prices to regain bear losses and move on to higher ground. (realdealretirement.com)