From AR Consulate London. About the only place I have ever found something in writing for tourists.
Length of stay allowed for travellers exempt from visas
On arrival, nationals exempt from visas are normally allowed to stay 90 days on a multiple entry basis. Before the expiry date they may apply for an extension at Dirección Nacional de Migraciones, only once and for no longer than the same length of stay granted in the visa. Thus, the new expiry date will be normally 90 days after the expiry date of the visa. If the person travels outside of Argentina at any time (under the orignal period granted on arrival or under the extension), when re-entering the country he/she will be usually given another 90 days, which can be subsequently extended. The number of times the traveller can re-enter the country in this way can not be determined beforehand and it is subject to Dirección Nacional de Migraciones being satisfied that the person is not covering an immigration purpose.
AR Consulate:
http://www.clond.mre...ar/en/node/2433