For most people, it takes an average of 6 months of trying in order to get pregnant.
Some people get pregnant very easily while others have to try longer.
There is such a thing as secondary infertility (difficulty getting pregnant the 2nd time around) for some people.
Conception can be more difficult the older you get (mid 30's) while others have no difficulties into their early 40's.
'Taking Charge of your Fertility' is an excellent book to read, and they have a site now that you should check out:
http://www.tcoyf.com
Although it's true for SOME people relaxing helps it happen, it's not true for everyone.
My husband and I tried for 4 years before we went to a fertility specialist who determined we had multiple problems to overcome and helped us have our son through IVF with ICSI.
We could NOT get pregnant without medical intervention.
At least we stopped wasting money on birth control - we didn't need it.