Size of the Gardens
The Gardens are 300 acres. To get an idea of walking times between landmarks see the Kew Gardens map (pdf). If you are visiting with small children be prepared to double walking times.
I avoided visiting Kew for a long time as I had imagined it was so vast I would not be able to see all I wanted to in one trip. I was amazed when I arrived and looked at the map and then looked at the landmarks and could see how close everything really is.
How Much Time?
It is suggested that most people take around three hours to explore the whole of the length of the Gardens. (It is about one mile across and takes about 40 minutes to walk across.) I usually stay the whole day and still never see everything but I guess we all have different priorities when visiting. I would suggest, if you have time, spend the whole day at Kew. Don't rush it; stay longer, have lunch, and enjoy your visit.
Kew Gardens is on the Heathrow Airport flight path. Noisy planes go overhead every few minutes. Initially this is distracting but, honestly, you soon get used to it and stop noticing them.
Kew is a photographer's paradise. You will see lots of people with cameras ranging from the cheap disposables to amazing long lenses on professional equipment. Most people walk round holding their camera and a map so if your camera has a neck strap do use it. As with any photo opportunity make sure you have all you need for a day away from the shops: extra batteries, film (if not digital), and an empty memory card or lots of space (for digital).