While I agree with The Wandering Dev Mar's answer in terms of what you'll spend your day doing, I think this is more a question about workplace etiquette, rather than day to day activity:
What your day will NOT be like
- Arrive at 8:50, make a cup of tea and take your coat off
- Sit down at 9:00 and work until 11:30
- Take a 15 minute break
- Work until 13:00
- Lunch until 14:00
- Sit down and work until 15:45
- 15 minute break
- Work until 17:30
- Go home
What your day WILL be like
Who knows. It depends on the culture of the company. As Dan said, your best bet is to mimic your co-workers, but I'd add that you should err on the side of caution: their job is secure, you're there to impress. You also don't know if they've got a medical condition which allows them more breaks etc. Generally, though, you're okay to mimic the majority of other staff.
This usually means things like
- If you go and talk to a colleague, you don't have to leave instantly the conversation is over.... if you stay for a couple of minutes to chat or joke, that's almost never going to be a problem. If you're stood around for ten minutes people may be bothered, but a quick chat is fine
- Take a break. This doesn't mean 15 minutes each morning and afternoon - if you need 5 minutes to stretch your legs and un-cross your eyes after searching for an escaped closing bracket, do so. It's better to take a 5 minute break and refresh your mind than to spend 2 hours trying to solve a 1 hour problem because you've got yourself in a bit of a muddle
- If you've got something to say, say it: a quick chat about current affairs when nobody is busy is a good way to foster team morale. Few companies will be concerned as long as you keep it sensible
And most importantly
Breaks are breaks
If you get in late (in which case apologise) or need to leave early (in which case ask permission) then certainly try to make up the time.
But if you need to take a few minutes to go to the bathroom, you don't have to take that out of your break time unless you're doing it every day (medical issue are the exception here).
Some companies allow smoking breaks, but you will usually find that you are expected to only smoke in your existing break times (and it's good practice to avoid non-smoking colleagues seeing you as a slacker)
For the most part, don't think of it like school or a rigid call centre job, this is a professional job: you're likely being paid a salary, not an hourly wage. This means that you're being paid to do "a job" not to do "x hours of work per week" - sometimes this means overtime as a deadline approaches, other times it means you can ask to go home an hour early.
You'll find a few companies are very rigid, and it's a good idea to take your cue from others in any case, but for the most part if you make sure you're getting your work done and aren't wasting excessive time, you won't even be noticed.