How often do you buy mailing lists?

[Joe Gee]

Hello, 

I've been wholesaling properties for about 4 years now. I still have lists from when I started. I haven't been as consistent with my mailing as I should be.  I know there are possible leads still in those lists, but also alot of activity has happened since then, and sellers had sold off.  

Any advice as to when to get a new list to mail?  Or should I just work the ones I bought 3-4 years ago?  

Thanks