I think it depends on the individual and how many books they really want to direct learn from.
I would look at putting in some ground rules for myself. Like applying DL to say 20 books over 4 days and then letting it go for 4 weeks and see what happens.
Since I want the DL experience to be productive I would avoid going to the extremes. (eg DL for 2 weeks and then not giving it enough time to let it happen.) In my opinion that would be counter productive creating an anxiety state. Personally I think 4 days would be the most I would do and then let it go for 4 weeks. My reason for selecting 4 days as the maximum is basicly because I wouldn't expect to spend much more than 4 days learning a single topic in a seminar and after 4 days I know from experience my mind is going ... all right already let me go and try it out. So for me it counter would productive to keep loading more of the same stuff on the brain. Probably I would realise when I have had enough anyway and stop when I feel I need to let it happen 
This will be a personal thing and probably depend on topic, desire, expectations and goals. Sometimes 1 day 5 books will be enough, sometimes we want more. The most important part is stopping to let the inner mind process it. Another reason for my selecting 4 days as a sort of maximum for a DL project is I want to be able to see results... I don't think much starts happening until you've actually let go and let the inner mind take over. If you are continuously DL when does the inner mind get to take over?
In short yes I believe you can DL from a number of books aver a few days and then let it go and see what happens.
Alex
