Keep you laptop well ventilated, and you can run it for whatever amount of time on AC. Laptop batteries have an average 900-1400 charge/discharge cycle life. Li-Ion batteries work best if they are not fully discharged regularly (i mean on a daily basis). I will recommend you to completely discharge your batteries thrice a month, once every 10 days, that is. This will keep them in good shape even when not in use.
@pathik: Removing battery is something that i won't recommend coz for one, you loose the benefit that laptop batteries provide, peace of mind from powercuts. If you loose power suddenly, you may loose data. Specially laptops are designed with always available power in mind. So that may hurt. Also charged batteries, even when put out, get discharged eventually. Secondly, the battery slots are not designed for frequent removal of battery from them and may get wear and tear easily.