It was back in 2013 when P&O Cruises refurbished Ventura, but she is due for another refit in 2018, which we can only presume will be a major refit. 5 years is a long time to be at sea, and so all that time will take its toll on the workings, along with the public areas.
She already had that refit back in 2013, which obviously helped bring it a little closer to Azura, seeing as though she is a newer ship. Azura recently underwent a minor refit this year, although was more of a wash and paint job.
People have already started to say that P&O Cruises Ventura is already looking tired, and for them the next refit cannot come soon enough, although this cannot be rushed seeing as though P&O Cruises already has other refits planned. You can see details of their next ship refurbishments here.
The next scheduled P&O cruises refit will be Adonia in May 2016. The last time she had a refit was back in May 2011, so would have been 5 years between those two dates, so the same as what Ventura will be between her refits.