I have been having irregular cycles ever since I quit nursing my 9
month old.  My last cycle was 45 days long - when do I ovulate??  Is
it still the 14th day after the first day of bleeding or is it more
like the 20th day since my cycles are so long??

I wouldn't say ovulation is per say the 14th day of your cycle. After
you ovulate you are then in the luteal phase which lasts anywhere from
10-14 days. The best way(or the one that works best for me) to
pinpoint ovulation predictor kits(OPK). They will detect your LH surge
which happens anywhere from 12-36 hours before ovulation.

There is also the basal body temperature method where you take your
waking temperature(with a basal therometer). Taking Charge of your
fertility is a book that explains this in great detail.

Also there's the fertility monitor which works for some women but is
the most expensive. The monitor costs around $200(off ebay a little
cheaper) but the sticks are $50 for a month's supply.

Lylabean

Ovulation normally occurs 14 days before your period begins, during
the luteal phase.  Since your periods are irregular you may be having
anovulatory cycles.  This just means that you are not ovulating yet. 
Breastfeeding can inhibit ovulation although it is not 100%.  It may
take some time before your periods become regular again.  This does
make it very difficult if you want to try natural family planning.  Be
patient.