1. How long are you supposed to meditate for (approximately)?
Nitnem is the basic must for all Sikhs (Soldiers of the Khalsa Army). Literally, nit = daily/routinely; nem or nee-yum = protocol/responsibility/duty. It consists of early morning (wee hours) bathing + 5 prayers (Japji Sahib, Jaap Sahib, Tav-parsaad Savaiye, Benti Chaupai and Anand Sahib — about 1 to 2 hours of sitting); sub 30 mins in the evening (Rehiraas Sahib); and finally one short prayer at the end of the day's battle/before going asleep (Kirtan Sohila). Amrit-vela (wee hours of the morning) attendance is pretty important. If you miss it, prayers still cannot be skipped. Except, they might be less effective for your mind. Amrit-vela (1 to 4am) and the evening 6 to 7pm are like the ideal seasons to sow the seeds of remembrance of Guru Sahib.
If your existing Nitnem (routine practice of meditation through Gurbani recitation) is successfully quenching your soul's thirst and if it makes you feel like you've won the battle before it begun for that day, you should look towards levelling up your game. Sangat (your company, the people you surround yourself with) plays an important role in this. Focus on the positives of others (while also being vigilant — pretty important. Ardaas, Chaupai Sahib and other protection seeking Banis help). Every day will be a new victory. There will be a fresh new bar every day.
2. Does anyone know any resources for learning Punjabi (so I can understand the service better)?
"Basics of Sikhi" channel on YouTube has pretty good resources for this. Some interaction with Sangat at Gurudwaras might also be of help. Some Gurudwaras also run courses/tuition for children that you might be able to participate in.
3. What are the rules on modesty for women, if any?
Manners and modesty expectations are the same from either of the genders. Dressing modestly for Sikhs, I assume, means to avoid flaunting their youth in a lustful manner. On the other hand, with Nitnem, Simran and Guru Sahib's merciful blessings, you'll have a spiritually radiating presence which will attract others to seek your Sangat. Stay focussed.
4. What is the difference for a women between wearing a chunni/dupatta vs a turban?
Personal preference. Majority of Punjab's Amritdhari women don't wear turban either. Ingrained from centuries' of practice, perhaps. However, some like to express their willingness to change their outlook to Guru's liking against the odds of "what will people think?". The same goes for Sikh men, for whom it is relatively easier since it's an established practice among them. Beware of stereotypical responses to this one. There isn't a strict requirement unless you're talking about going into full scale seva in Guru Sahib's presence (multiple levels of meanings intended), which might be with a Chola and a Turban. Do as your Panj Piyares (ones who bestow you with Amrit) let you.
5. Are there any versions of sri guru granth sahib available in English?
I believe this has been answered before. However, you should introduce yourself to Gurmukhi script, Punjabi language and literature, as well as other go through Katha (discourses) of Gurbani. Online English translations only go so far. Gurbani in its original form is way personal and soul quenching. To be very honest with you, for someone who hasn't been introduced to Gurbani at an early age (even if you're from Punjab) it is hard to get a grasp on what Gurbani says the first day or even getting started with reading the harder Banis. I suppose you're just one step behind (viz. knowing how to interpret the script — correct English "transliterations" might help). Repeated recitation (as part of the Nitnem) encourages you to rethink the meaning afresh every day. Getting acquaint with just one word a day is soul cleansing. If you have an iPhone, "Daily Hukamnama" app by SikhNet might also be of help.
All the best.