Can itc reversal under rule 42 of cgst act be done half yearly .

Rule 42(1) says exempt supply shall be determined provisionally for each tax period and Input tax credit shall be reversed  specified in sub rule(1) .

( Please refer rule in detail, I have written in short) 

So the word Tax period is important here. 

And Tax period  is defined under section 2(106) - Tax period means the period for which return is required to be finish.

  so return under section 39( GSTR3B ) required to be furnished  either monthly or quarterly. 

So considering above section & rule you have to reverse monthly. 

Do more interpretation & take others opinion as well. 

Also I would like to add in the above reply rule 42 is  applicable only to Input & Input services not for Capital Goods, rule 43 of the CGST rule2017 , applicable to  Capital Goods.