Mike, I would never spend a lot of time doing submissions - that is my opinion. Sitting there and trawling through a chunk of the list is not only monotonous, its not a very productive use of your time. Directories are but ONE method to build links. Some people rely solely on them, and they are an attractive method simply because they are dead easy and require no interaction or negotiating. What I suggest is that you spend a little time each day, or more time on just 1 day, or whenever you have a spare 15 minutes say, to knock over another 4 or 5. I should be careful here, because I also suggest getting in whilst the directories are free (before they convert to pay-for's and reciprocal required). Timing is everything I guess. If you can spare the time to do it, then do more... For my main site, the site itself is up-to-date with directory submissions so I only need to submit it to the latest batch of 20-30 each time I do an update. That obviously makes it a lot easier. If you have a lot of sites and find yourself in a never-ending catch-up mode, then maybe it's time to try one of the submission services. There are stacks of DP users on here that offer them. You'll have to do a search and find them. Acceptance is an entirely different issue. That all depends on the directory owners diligence, and the particular directory's age and popularity. General rule of thumb is that new directories (which usually have a low PR too) will accept quickly (within 24 hours to a few days), whereas more established directories (which receive many more submissions per day) may take anywhere from a week to several months.
silencer, thanks for a detailed answer. In fact I only submitted to few directories so far, those which have the highest PR and which are related to my sites' theme. I agree with you on it, of course there is no need to submit your site to all new directories. The way you described it seems to be the best.
Whether or not you pay a directory submission service to submit for you would mostly depend on whether you are time poor or cash poor. If you have the time, do it yourself; if not, pay someone else.