"put in extra time" is correct and usable in written English.
It is used to refer to putting in an extra effort or spending extra time on something, usually to improve it. For example, "I plan to put in extra time studying this week to prepare for the test next week."
Bank boards met more frequently during the financial crisis to plot strategy, and Citigroup's directors put in extra time last year in their discussions over whether to replace Vikram S. Pandit, the bank's chief executive.
Williams and his teammates have put in extra time lately with the assistant coach Doug Overton.
Students actively helped other groups and put in extra time at lunch and at home to develop their apps.
Perhaps not the most gifted athlete, but one who would put in extra time and effort to improve.
Although the press has played up the company's 35-hour workweek, the truth is, employees often put in extra time to complete a project or fulfill a responsibility.
They said Volkswagen had made a big concession on its demand that workers put in extra time if they failed to hit production targets.
John is showing outward signs of engagement by putting in extra time with clients — but could he be a Workhorse or a Martyr who is suffering in silence?
Dustin Brown was putting in extra time, working hard after the Kings' morning skate the other day.
Kevin's BATNA was to stop putting in extra time for the other division and return to his "day job"; his colleagues and boss had weaker BATNAs, because they had nobody as well suited as Kevin to do the relevant work.
Because he was seen as putting in extra time and thought at the lower level, and therefore having the potential to grow into the higher level position".
When it comes to football he puts in extra time to learn and understand his assignments and it is not a problem.
