As someone who works in finance, I can tell you it's going to be less than a year. Although uncommon, those things sometimes reboot as part of maintenance. For example, we upgraded our COBOl compiler and all got an email that for like half an hour after midnight in a weekend the mainframe would be down for maintenance.

We rebooted system as part of training every two years, new staff needed to know how it is done :-)

I'm a Natwest/RBS customer. I'm going to guess their uptime is about three weeks.

My experience is literally that 1/3 of my logins result in "our system is down right now". Probably because I often want to check my account in the evening on a weekend.