Is it possible to change credit limit from amount to a certain amount of time…e.g days, weeks or months for payment due…
Because for some companies either sales or purchase they give like 2 months or 3 months credit…time for payment instead of credit amount…
No. The optional Credit limit field is referenced in financial calculations showing available credit amount. You cannot mix time with money.
Amount of time available for settling invoices is listed under Due date. If you have different terms for customers or suppliers, enter that in the Additional information field when defining them.
Where can I find the due date …
Right at the top of the entry screen for a purchase or sales invoice:
Your supplier sets the due date for a purchase invoice. You set it for a sales invoice. I suggest reading some of the Guides before posting questions. These things are all explained there.