How long does it take to fork a dash coin to something like PIVX ?
Depends what elements of Dash you want to fork with it.
Dash currently has :
POW mining (X11) (miners get 45% of the block reward)
Masternode network (masternodes get 45% of the block reward)
Decentralized Budget System (10% of the blockreward is reserved for this)
Decentralized Governance System (masternode owners vote over budget proposals)
InstantSend (optional : transactions can get confirmed instantly)
PrivateSend (optional : transactions can be sent in a private way, which make tracing them on the blockchain incredibly difficult)
DAO Setup (Decentralized Autonomous Organisation)
and Dash is working towards a major update called Dash Evolution, which extends above elements with new elements focussed on userfriendly and scalability.
In order to become a successfull fork from Dash a cryptocurrency needs to have all of the above and also have them improved and extended in some way.
I think that will be very difficult to accomplish. Already there are tons of Dash forks out there, so the competition will be brutal.