You could theoretically arrange for them (by professors) any time, by that I mean, talk to them about it, they agree, etc. But I'm fairly sure the professors/dentists have to send in their recommendations directly to AASDAS during the application cycle you're applying in (June to Feb or whatever it is). So it would be impossible for you to have them too early. Bottom line, if you can arrange your recommendation letters now, do it, then when your application cycle comes, just speak to the people you asked for recommendations and have them send them in.