Unfollowing after a certain number of follows, not days, works best for me. Removing accounts with a follow back ratio of less than 0.2 after 25 follows or so could be a good starting point. You should check back regularly and sort your sources by follow back ratio from highest to lowest. If you have a lot of accounts above your threshold of 0.2 then raise it to 0.25, 0.3, or whatever number would still leave you with a healthy variety of sources. If lots of accounts are not above the threshold, consider lowering it to .15, .1, or adding different sources similar to your source accounts with the highest follow back ratios.