Minimum 7 days notice of resignation even though you didn't receive any offer letter. This one of process of handover and also to respect current employer who support you during your less 3 months employment. In future, any company you attach, please get official offer letter first before join any organization (with their signatory also your signatory too). This to avoid any Industrial Relation case also proof that you working with them.

if you're still under probation, I recommend for you to actually give 7 day notice for a proper way resignation, so it's enough time for you to do handover.