Schools vary enormously in how long they will hold an offer open for a candidate. Some schools will give exploding offers, requiring a candidate to decide immediately or within days. If you receive an exploding offer, contact the Law Teaching Placement Committee and your recommenders for advice on how to proceed.
Most schools will give a candidate 2-4 weeks to decide whether to accept an offer. However, they are unlikely to allow more time, since doing so would diminish their ability to hire their next choice, should you decline their offer. Ask for what you need, but be ready to read the tone of the response. Consider offering to withdraw some of your other applications or decline some of your other offers, if the offering school will keep the offer open a bit longer.
Remember that it is acceptable to negotiate the terms of your employment—after you have an offer. Many candidates are successful in negotiating which courses they will teach, a reduced courseload for the first year, a research stipend for one or more years, more generous moving expense coverage, and other issues. The Law Teaching Placement Committee can be particularly helpful in giving you advice about which issues might be negotiable and how to approach negotiations.