This may be from day one - if you are engaged on a project and you know from day one that your involvement will last three years then you get nothing.
If you are engaged on a project and your involvement will last 18 months then you are fine unless, say, after ten months you find that the job is so far behind schedule it is going to take 25 months. As from that date you cannot claim.
If you are engaged on a project and your involvement will last 25 months then you cannot claim. If after ten months the project is two months ahead of schedule then the finishing date is 23 months from the commencement. You can then claim as from the ten months (but not for the first ten months as the likelihood at the time was that it would exceed the 24 months).