You type this command directly into twitch chat and it should work:
!addcom !howlong $(touser) Has Been Following $(channel) On Twitch Since $(customapi http://api.newtimenow.com/follow-length/?channel=$(channel)&user=$(touser))
$(channel) is replaced with the channel name wherever it is ran
$(touser) is replaced with whatever name is used immediately after the command and defaults to the person who ran it if none is provided.
If you want it to be restricted to a certain userlevel you would add -ul=[userlevel] after !howlong