Sparkly – to answer your question: Girls don’t have to worry about zman Krias Shema. They do have to daven by zman tefila.
Ideally, you are supposed to daven by one third of the day. The way that is calculated is by figuring out how much time there is between Netz and shkiya (it’s different every day), you take that amount and divide it by 3. For example, let’s say that netz is at 6:00 am and shkiya is at 6:00 pm. That means there are 12 hours between netz and shkiya. 12 divided by 3 is 4. So you add 4 hours to netz, and you get 10:00 and that would be the time you are supposed to daven by, l’chatchila.
HOWEVER, if you didn’t daven by then, you can daven by chatzos and that is most girls do. You can find chatzos by looking in any Jewish calendar that gives zmanim. You can also find it online, although I don’t know which websites are accurate – I’m sure there are others here who might know. If your calendar doesn’t give the time for chatzos, but it does give Netz and shkiya, you can calculate what halfway between them is.
Technically, according to the Mishna Berurah, you must finish shemona esrei by chatzos. The “minhag” amongst many girls is that it’s okay to start shemona esrei by chatzos. I am not sure of the source for that and I actually would like to find it if anyone knows if and where it exists.
If it is getting late and you don’t have so much time to daven, this is what you are chayiv in (as per R’Elyashiv): all of brachos and birchos hashachar, parshas hatamid, baruch sheamar, ashrei, yishtabach, the first sentence of Shema and if possible the first paragraph, Emes v’yatziv until ga’al yisrael, and shemona esrei. And then at Mincha time, you are chayiv to say shemona esrei. On Shabbos and Yomtov, you also have to say Nishmas.