I think a general rule of thumb would be about 150,000 miles, but personally I would only change it if there is a problem. A problem will be indicated by small leaks which should be obvious (you will see coolant on the ground and coolant levels declining), or by the pump making a lot of noise. If the temperature malfunction light comes on however, you should stop driving the car.
Changing out your water pump on your own is a very difficult job with the Echo, see other answers for how it is done, but I recommend you pay a professional to do it. The cost should be around $300 for parts and labor (the pump itself is a $25-$40 part). It can take several hours to complete this job.
yes
In the gas tank. I have 2001 Echo.
2001 Toyota Echo doesn't have a timing belt.
http://www.carspace.com/guides/Toyota-Echo-How-to-change-oil
http://pdftown.com/2000-2002-Toyota-Echo-Service-Manual.html
3.5 Liters
see link
timing chain.
32 PSI
go to dealer
i have a 2001 and it gets about 39-42 mpg
See related questions.