Most schools adhere to this, although by summer a school may want a more immediate reply. If you have another interview scheduled or are waiting on the outcome of one, you may have to negotiate for a longer period. Thus you may have to consider whether the bird in hand is worth a better one in the bush. Get advice from your major professor and the Placement Director among others before you decide. Likewise, if you have two offers in hand or have one with another interview scheduled or offer anticipated, you may want or have to do some negotiating about salaries, etc.
Strategies vary depending upon circumstances.
Get advice on this also.