How often should I update my portfolio?
Your portfolio needs to reflect your current look. Since hair styles and clothing styles change rapidly your portfolio needs to reflect not only your current look, but also the current look of Fashion.
Advice from Igor F. Malkov:
"Models - aspiring or experienced all the time asking me the same question: How many pictures suppose to be in a good modeling portfolio?
The answer is simple but listen carefully - it is very important: Never show your professional limit! What does it mean? Never include bad or not very good photographs in your portfolio. If you have only 2 good photographs you should include only 2. If you have 10 good - include 10.
You can find this advice:
"Keep your best picture at the end of your portfolio to leave a lasting impression."
What do you think about this advice? Is it good? I do not think so!
Please understand one thing: If you have, for instance, 4 BEST photographs but included in your portfolio also several not so good - you show your professional limit.
Just read carefully: ONLY 4 PHOTOGRAPHS ARE GREAT!
But if you included just 4 EXCELLENT photographs - the result will be completely different - ALL PHOTOGRAPHS ARE EXCELLENT!
If you don't understand what I'm talking about, go back and read again.
It is a best scenario if modeling portfolio included not only photographs but also many tear sheets showing your previous work. Each client want to hire experienced professional model. That is why tear sheets in portfolio so important for getting job. But please - not each tear sheet can be good for portfolio. Always consult with your agent or manager before you include any photograph or tear sheet in your portfolio."

Recently many photographers use digital cameras for shootings modelling portfolios. In these cameras the film is not used. Selection of photos can be made immediately after shooting. The selected photos can be both are printed, and written down on CD for printing or viewing on the monitor of a computer. Always preliminary stipulate with photographer how many photos will be written down on a disk and how many will be printed.

Important point.
Never criticize your own pictures. If you think your photos are bad you shouldn't be wasting time showing them. Let the agent give you his own professional opinion.

One interesting subject.
How good are photographers shooting for free?
Good question but there is no answer. Some of them are good and some of them are awful. Sometimes photographers shooting for free, if he shooting commercial job and doesn't want to pay to models. If you are shooting a free test and photographer asking you to wear something what is not belongs to you - it may be a commercial shooting. Read very carefully modeling release.
Sometimes free shooting means that photographer is just a beginner and needs models to create a photographers portfolio. It's OK for getting experience and photographs for your initial - I have to repeat - only initial portfolio.

Mandatory: Photographer must be familiar with the requirements of the industry.  Otherwise you can get a lot of bad and useless pictures, bad experience, wasted time and money.
All you have to do is practice! If you will practice how you should, you can become not only equal partner to photographer but even can guide some of them. But anyway, it is always a good idea to shoot with a good fashion photographer.