I think I spent a good 2-3 months just on stage 6, just to practice that F. The other thing I told myself I'd do is try to always use the barre F and not the "mini F", because using the barre F makes you practice it in songs and that helps getting better. If you use the mini, you aren't practicing the barre in songs, and then how will you ever get it? I'm on stage 9 now, and after playing the barre F for 6-7 months, I can choose to use the mini-F now if I want. One thing I did over and over is the All Along the Watchtower chord progression with the strumming pattern shown in the book. I'd just sit and do it over and over, and that got me practice at going between the F and and Am and G, plus a cool strumming pattern. I go back to that as a little warmup sometimes. You gotta do the 1 minute changes for all the F-changes, a lot. I really did those for months, until they were pretty much all well over 50 per minutes (The C-F over 60). Some people get the F really easily, for others like me, it has taken months, and there is still room for improvement! (My pinky is unusually short, like only halfway between those 2 middle knuckles, so I have interesting challenges in just getting my pinky to reach, in addition to the barre!). I'd say move to the next stage when you can do F-C, F-G, F-Am, F-D all preferably over 50. Then just doing some of the F changes each day, rotating which ones need work. Let It Be by the Beatles is a good practice one too, it goes C-G-Am-F-C-G-F-C.