Neither.
Freepost is treated completely differently to 1st or 2nd class mail and there are different processes that take place.
However, it would normally be delivered in about 3 - 5 days dependant on the arrangements put in place by the receiver. (ie direct daily delivery, or regular collection)
Also, some departments within DWP have extra processes in place for Royal Mail which could either add to or reduce the delivery time.
In effect, Freepost delivery speed depends on a lot of factors so there isn't one generic answer to your question.