Length of fertile period?

Hi, I've been TTC for #1 and I used to use OPK's but stopped using them as I didn't think they were v.accurate and they're too expensive! My cycle is regular at 27days and I'm tracking my ovulation through this app. A friend told me to keep having sex every 2nd day once my period had finished for 2 weeks as ovulation might not consistently occur on day 13. So I'm going to try and continue up until day 20. Does anyone else do this or advise me whether this correct? Thanks!