I've never heard of all readings everywhere having a set validity.
Personally, I'm a strong believer in personal choice influencing your future (in most situations I can imagine) so I don't have a proper time frame. I just assume it's about the near future and tend to read it as the most probable outcome.
For example, if I'm reading about a friend's life within the next few months and there's something about that person accomplishing something positive at work and their current relationship ending, there's a higher chance that some of it is due to current factors. The work accomplishment could be because the work they're doing now will pay off and the end of the relationship could be due to troubles that are already there. I think it could still be possible to change that outcome, if the person decides to leave their job tomorrow or address the problems in their relationship.
If I'm reading about my friend's life 50 years now, while some things may be because of factors already at work, there's much more time for that person to change their habits and fix patterns in their life. I can do a reading about that and find out that friend will have lots of health problems. It's possible that some of those health problems are because of their current habits and can't be avoided, but the chance of that person being able to change things by getting healthier habits like eating better, quitting smoking, and so on is also much higher.
That's my opinion, at least.