Hello Pawan,
It depends which upgrade you are performing.
If its only Netweaver upgrade/Java upgrade then 6-8Gb is sufficient.
But if system is ERP system and you are moving from EHP5 ERP 6.0 to EHP6 ERP 6.0 then i have experienced that SUM folder is big chunk of 14-20GB after upgrade.
Generally as per rule of thumb recommended free space is 2.5 times the size of Downloads(files which are upgraded in upgrade). I am talking only about SUM folder not DATA and LOG folder
Regards,
Surojit