I haven't seen any blog posts about this yet, so I figured I'd share.
Planting each herb with the fertilization of 2 guanos yields the following times:
Purple = 3 minutes
Gandlevery = 4.5 minutes
Rubeweed = 12 minutes
Hairball = 13.5 minutes
Rookswort = 15 minutes
Silvertongue = 21 minutes
Yellowcrumb = 27 minutes
Guano takes off 90% of the remaining time required for herbs when 2 are used, and 50% when one is used. If anyone would like to know those times I can post those as well.
Thanks to Lx for correcting me ( :P) Hehe.
Hope this helps!