As long as there is not a lot of anaerobic work being done, one could probably do forms of speedwork quite often. Since most races over 5k require mainly aerobic functions, I tend to key workouts in that field. If your speedwork reaches anaerobic levels too often, then your aerobic training/ability could suffer.
Because of this, I tend to (or try) only hit anaerobic levels 2-3 months out of the year in training. That's because I like it. Other times of the year I tend to try and do work close to what I want my race pace to be. Surprisingly, you don't need much of it (anaerobic) to accomplish what you need as a distance runner. It should be a small percentage (maybe 3% max?) of yout training.
For marathon training, I work on bringing my pace down on training runs considerably close to my goal pace as much as I can handle. This is why it is difficult for many to perform well all of the sudden when they have 2-3 months to train for a marathon. First you lay the ground work and then set aside a nice block of time to work on maximizing your goal pace for race day. When the marathon comes, you should have a very good idea on what pace you can carry. As far as the amount of pace work you can do a week, I find this to be very different for each person based on factors, such as years of training, overall leg speed, weekly miles, dedication, intelligence, design, age, and ability to recover/rest.
So if your definition of speed means running close to goal pace aerobically, then you could do it for a considerable stretch of time. If it means blasting out sessions a couple times a week an going anaerobic each time, then not much more than 2 months or so, especially if your racing during that time as well.