Delete a row after 1 month or 3 month based on selected validity

Basically you must delete obsolete rows where current date exceeds date_of_entry plus validity days in this way:

now() > date_add(date_of_entry, interval validity day)

create table tbl (id int, date_of_entry datetime, validity int);

insert into tbl values
(1, DATE_ADD(NOW(), INTERVAL -31 DAY), 30),
(1, DATE_ADD(NOW(), INTERVAL -29 DAY), 30),
(1, DATE_ADD(NOW(), INTERVAL -92 DAY), 90),
(1, DATE_ADD(NOW(), INTERVAL -89 DAY), 90);

delete 
from   tbl
where  now() > date_add(date_of_entry, interval validity day)

select * from tbl;

id | date_of_entry       | validity
-: | :------------------ | -------:
 1 | 2018-03-11 18:16:43 |       30
 1 | 2018-01-10 18:16:43 |       90

dbfiddle here

In MySQL you would do something like:

delete t from mytable t
    where date_of_entry < curdate() - interval validity day;

This assumes that validity is either the number 30 or 90.

DELETE FROM mytable WHERE date_of_entry <
  DATE_ADD( CURDATE() ,INTERVAL validity*-1 DAY) ;