My favorite Timeful feature (which I hope becomes a Google Calendar feature) was that you could set time estimates for to-do items
What I learned about productivity while reinventing Google Calendar
David Kadavy

Yeah it would be great if I could specify this goal is probably about 90 hours work and gcal then goes and finds slots totalling 90 hours. This would allow me to get a better look at my mid term calendar to see what commitments I may have