Right now, a good systems person can expect more. Indicate your appreciation upon receiving an offer, but if you need some time to think it over, ask for a few days. You should generally be able to give your answer within a week. If you are going to take longer because you have other situations or circumstances, tell the company what your problem is as soon as possible and see if they are willing to wait. If the offer is in the ballpark and you are going to seriously consider it, ask if the prospective employer would send a letter summarizing the major points of the offer.
Many companies do so without being asked.