How long can Netflix downloads remain on your phone? You won't find out, not until it's too late.

Subscribers have been asking for offline Netflix access for years, so they can download their favourite movies and TV shows to watch when they're either offline or stuck on a connection that's too slow or expensive to be practical for streaming. Netflix has finally delivered offline access for Apple and Android devices, but the fine print gets rather messy.

Don't assume you can download your favourite episodes of your favourite shows and keep them on your device indefinitely. Some shows aren't available to download and those which are expire after a while, but there's no way to know up front how long you'll have access to a particular movie or TV show.

Netflix's explanation of download expiry limits on its FAQ page is frustratingly vague;

The amount of time you can view a title depends on individual licenses and can vary by title. Titles that are expiring from your device in less than 7 days will display how much time you have left on that title on the "My Downloads" page of the Netflix app.

For some titles, viewing offline must be completed within 48 hours of starting the play. Once you have started playing one of these titles, you will see how many hours you have left before the title expires on the "My Downloads" page.

If you can't keep downloads on your device indefinitely then Netflix offline isn't very practical as a "break glass in case of emergency" library, keeping a few episodes of Archer on standby for those times when your flight is delayed. Instead, the download option seems to be intended as a short-term feature, requiring you to plan ahead when you expect to need it in the near future.

The problem is that the expiry limits aren't listed alongside each movie and TV episode, so if you're planning for a long trip there's absolutely no way to know how long the movie or TV show you're downloading will stay on your phone.

Sometimes the expiry timer starts from when you press play, but again there's no way to know this up front — which is especially frustrating if you're downloading content for young children who tend to want to watch the same things over and over. As it is, you're in danger of wasting a lot of time and data downloading offline content that's still going to leave you in the lurch.

After chasing Netflix for more details I learned that all downloads expire after 30 days, even if you haven't watched them — something which isn't included in the FAQ but that Netflix should have explained. The 30-day expiry date isn't marked in the download list, you're just given a seven-day countdown as the date approaches, which makes it more difficult to manage your offline library before a long trip.

What's even more painful is that you'll only discover which videos have a 48-hour limit after you download it, press play and then hit pause, which is likely to catch a lot of people off guard.

If you're feeling cynical you might suspect that Netflix fails to mention the 30-day limit because it wants to look more flexible than Amazon Prime – which is upfront about its 30-day limit on offline content.

Netflix's FAQ page explains that downloads aren't automatically deleted off your device and it's possible to renew them without downloading them again. Well, at least sometimes;

If a title has expired, you do not need to download it again. Simply connect to the Internet from your device to renew the download from the Netflix app. Some titles may only be renewed a certain number of times.

Due to studio limits, you may not be able to renew all titles. If you are unable to renew a title but would still like to watch it, you'll need to connect to the Internet to stream.

Once again there's no way to know any of this before you download a show. As far I can tell you're not even told after you download it. You won't discover the offline limitations until you hit them – when you probably can't do anything about it.

In Netflix's defence, all these hassles aren't its doing — they're due to the frustrating nature of archaic rights deals, a concept which the Netflix Originals strategy sets out to smash. People sign up for Netflix to watch television on their own terms and to get away from this kind of arbitrary crap.

The streaming giant might not want to draw attention to these hassles but it needs to do the right thing by its subscribers and tell them up front so the download feature doesn't disappoint.