I’ve been using Express VPN for quite a few months now and have almost always left it on all of the time on my iPhone. I’m my experience it’s perfectly fine to do this. I’m using an iPhone 11 with it’s great battery life and was concerned that it would drain it a lot quicker but even with it enabled but in my testing I can still go 2 days between charges.
In the last few days I’ve reported 20% background use as shown above.
I’m also not seeing a lot of difference in performance. I’m watching video, streaming music and even running Xbox Cloud Gaming all fine. I do experience the odd site that won’t load due to the VPN but I just stop going there if that happens.
I’d say you can leave your VPN on all of the time to maintain your privacy and surf in the comfort that your ISP isn’t going to be snooping on your data.