Now that the variable deadline system is operational, I have lowered the deadline for SR5 to 4 days, which is appropriate to tasks of this length.
If you are lucky enough to find a prime (which needs extra processing) the deadline extension system will increase the deadline as needed up to a maximum of 30 days. This also applies if you're just running a very slow computer. With the maximum deadline of 30 days, the deadline is effectively twice as long as the original 15 day deadline, while simultaneously allowing the server to be able to send out replacement tasks much faster if a computer goes missing without returning a result.
My lucky number is 75898524288+1