I have a 26 weeker, and she was on a vent for about 3 weeks (a regular vent and then an oscillating vent) before she transitioned to Bi-pap and Cpap. She did get a pseudomonas pneumonia infection because she as on the vent about 10 days after birth and that is one of the reasons why they had to keep her on the vent. She ended up coming home on oxygen (after 83 days in the NICU) and weaned off slowly off the oxygen (first 24/7, then only needing it during meals and sleep, then just sleep, etc). at the end of July. She was on oxygen for almost 6 months.
However, keep in mind that my baby was younger than yours and that makes a big difference. She may not need oxygen for that long. I hope that your baby turns a corner and that if it is an infection they will have caught it early. Keep us posted.