Hi, welcome to HealthcareMagic. There is no thumb rule for getting pregnant. Sometimes prolonged intercourse is necessary, while in other cases even a single intercourse may result in pregnancy as in rape. The timing of intercourse is important in this respect, 7 days prior to and immediately after periods is considered Safe period, that means intercourse during those days does Not result in pregnancy. However many other factors affect conception: whether there is ovulation, whether tubes are patent, whether the male ejaculate has sufficient number of motile sperms,etc. You need to be investigated for these parameters, do see a gynaecologist. Further, I'd like to remind that you are in your twenties, hence you have time on your side. Do not worry unnecessarily. Hope this information is useful.