If the offers provided by buses do not suit your needs, trains and carpooling could represent a good alternative for this distance.
Despite being extremely cheap, the bus is also very slow and getting from Warsaw to Madrid by bus will take you a while. You could consider travelling by plane, getting low-cost tickets if you can book in advance.
Travelling by bus to Madrid from Warsaw: how many daily connections are there?
When are the buses from Warsaw to Madrid scheduled?
The first bus leaves from Warsaw at 11:10, while the last one leaves at 11:10. The journey takes 44 hours and 45 minutes.
By bus from Warsaw to Madrid: which companies offer the service?
With Sinbad-Interbus, Sinbad and Sindbad you will be able to travel trom Warsaw to Madrid by bus.