There is only one way to get pg naturally. Have sex. Every second day from day 10 til your period comes. if you have a 28 day cycle, that's only 9 shags, so not too much effort. If you are able to pin point the day you ovulate you can shag more frequently round that time, but definitely keep going for a few days after. The first time I got pg we just shagged 2-3 times a day for 10 days, we were on honeymoon, so pretty easy to do that! The second time it took 7 months, the last 4 of which I was using CBFM. That ended in a mc, and we are trying again and on cycle 3. I am using CBFM, charting temps and CM.
Oh doh, that doesn't answer your question.....
To maximise your chances you need to know when you are going to ovulate in advance of the event, and then confirm that it has definitely happened (because once it has you don't need to continue SWI). You can predict ov'n using OPKs, checking CM, saliva and your cervix, and with CBFM. You can confirm ovulation by a sustained thermal shift when charting BBT. Have a look at the Fertility Friend website for more information. If you are very regular, charting alone will give you enough info, but if not, the extra information from OPKs etc is reassuring.
Armed with this info, you shag every second day until you get a positive OPK, a high (or a peak if no highs) on the CBFM or you notice EWCM. Then you go daily till your BBT chart confirms ovulation. I sound like an expert....too much googling!