When Arrow first premiered in 2012, the series introduced a secondary timeline set in the recent past of protagonist Oliver Queen.
If each season of Arrow is roughly a year in the life of Oliver Queen, then it stands to reason that the flashbacks, with some exceptions, constitute a similar amount of time.
“This is probably the last year of the flashbacks, which means that we have a lot to accomplish.
Essentially, it could be said that after season 5, the past will be firmly in the past.