All the good advice seems to point to 300mg/day as the magic number (from the World Health Organisation), which translates to about 8 shots of espresso per day. I know only a few people who would normally have this many shots in a single day. Drip (brewed) coffee, if you're into that kind of thing, is much less caffeinated - less than half. The problem being that servings of drip coffee are much larger than the typical one-ounce espresso shots. This brings the upper daily limit of drip coffee servings to around 3 x 8 ounce cups per day.
Now obviously, instant coffee is another kettle of fish (just because life loves making everything more complicated). The coffee that we get is made for the Asian market, so has no advice on the label whatsoever... The internet, however, tells me that there is approximately 32.5 mg caffeine per gram of instant, on average. How does this translate? Well, if you're pouring the stuff in by the fistful, you're going to have a bad time. If you like it on the lighter side, though, you should be fine to have a couple of mugs each day.
Just recently (three days ago), a Scandinavian study has been released that identifies caffeine as a cause for low birth weights & smaller babies. This news might come as a boon for some mothers, but for their offspring, the results can be ominous. Coffee was found to prolong pregnancy, but overall, excessive caffeine consumption was found to increase risks of pre-term delivery and tinier babies. You can read the science here: http://www.biomedcentral.com/content/pdf/1741-7015-11-42.pdf
Christopher's conclusion: switch to tea, and then treat yourself to a bit of dark chocolate if you need a pick-me-up. It's heaps better for you so you don't have to feel guilty - apparently women do that...