How regularly you should take backup of the computer data?

Depending upon! personal or commercial and volume. However, either it must be taken at least once in a day or if possible then on real time basis.

If it is server data, it should be on daily basis. However, this will depend on IT policy of the organization. 

If you back up after long intervals a lot of data will be lost in case of any mishaps, accident or fire. However, 

full agree with answer given by Mr. Fazlur

At the END OF DAY. And at the month delete all other backup and just keep the last backup of the month.

Everyday, before your knock off.

once in a month needs to have a back up account 

Backup should be everyday if possible or once a week.

Thanks 

Every two months is fair enough to backup according repetition 

Backing up to an external disk or drive

Using a third party back up service

you can deploy daily/weekly or monthly backup. but it depends on the organisation data storage policy.