In the early days of the link building process and based on my own early works for so many clients in doing manual directory submission, there is totally no limit whatsoever on the number of submissions that a link builder can make for a client's website. There is no rule of thumb when we are talking about the limits of the the approvals that a link builder can get from a days work combined to all other link building efforts such as blog comments and forum postings.
Today, link building using link directory submission seems to have a dramatic change. Google seems or in some cases have implemented penalties for those who have been able to get as much links as they can just like in the older days. This is very different from what I have known being a little over 4 years in the link building arena. It's also a good chance for me not using automated submissions since and not going for reciprocal links for my clients since.
The Google penalty indeed changed the nature of a link builder's work and it has favored those who use and implement good link building habit like me. I never get in to automated submissions and never got in to reciprocal linking and I have benefited from that process since and my clients got no complaint except for some occasional algorithmic changes to the organic results which is a natural phenomenon.