If I have an invoice that does not contain a payment schedule, when am I obligated to pay the invoice?

To determine when a payment is due on an invoice, it depends primarily on what product was sold to you or what service was provided to you.  Generally, if a product has been delivered to you or a service has been performed for you, then the invoice is due and payable upon receipt.  Some invoices may indicate "Net-30" or similar language -- which language would indicate that the seller expects payment within 30 days.  The best thing to do is to contact the seller or service provider to inquire when the seller or service provider expects payment.