If you are getting married as the second wife, how do you or your husband decide if you have your time first and how many days you get each time?
The man who has more than one wife must maintain practical justice between his wives. He must organize his time between them to be sure that each of them gets equal rights.
It is his responsibility to achieve that practical justice, not yours as a wife.
If the man is unable to maintain practical justice between more than one wife, he will be then not allowed to have more than one wife (But if you fear that you shall not deal justly with wives, then just one wife. ( Sura Al-Nisaa', verse 3)
Wassalam.