I went through a few pages of cases, where the victim was found murdered, on the Located forum, to see how long it took until an arrest (if any):
Arrest within 2 weeks of victim being reported missing
Jorelys Rodriguez: 5 days until arrest
Lauren Giddings: 9 days until arrest
Skyler Kauffman: 2 days until arrest
Tyler Dasher: 1 day until arrest
Yashanee Laface Vaughn: 12 days until arrest
Lauren Ackerson: 7 days until arrest
Sian O'Callaghan: 5 days until arrest
Mariha Trenice Smith: 13 days until arrest
Breann Rodriguez: 7 days until arrest
Unsolved
Celina Cass: unsolved for 4 months so far
Sonia Varaschin: unsolved for 15 months so far
Morgan Harrington: unsolved for 2+ years so far
Sylar Newton: unsolved for 15 months so far
Phylicia Barnes: unsolved for almost a year
Kera Freeland: unsolved for 7 months
Lachlan Cranswick: unsolved for 10 months
Kelly Catalano: unsolved for 10 months
1 month or longer until arrest
Michelle Le: 3 months until arrest
Alexis Rasmussen: 2 months until arrest
Kenia Monge: 4 months until arrest
Mackenzie Cowell: 7 months until arrest
I know that Lisa hasn't been found, and this isn't scientific research, but it does seem like if an arrest isn't make relatively quickly in a case, it's going to go unsolved.
Sometimes, I wonder if a case getting its own forum is a bad omen. It seems like if a case gets its own forum, it's because it's expected to have continuous interest, and that's more true of unsolved cases than cases that are waiting to go to trial.