That is such a complicated issue, we cannot give a definitive answer here. However, there are some factors which can be mentioned here.
The earlier the child is gets appropriate help (and not any and all help is appropriate), the less time will be required. A huge study by The National Institute of Health found that of the children who were behind in 3rd grade, approximately 75% of them were still behind in the 12th grade. In order for a child to get out of that group, it is felt that massive amounts of intervention are required. However, children who are identified and given appropriate interventions in kindergarten, first and/or second grade fare much better.
The brighter the child is, other things being equal (and, of course, they never are), the less time will be required.
The more highly motivated the child is, the less time will be required.
The more highly skilled the teacher is, other things being equal (and, of course, they never are), the less time will be required.
In general the more intensive the amount of help the child receives IF IT IS APPRPRIATE, other things being equal (and, of course, they never are), the less time will be required.
The more the appropriate the help is for that particular child, other things being equal (and, of course, they never are), the less time will be required.