The Los Angeles Times unspins it for you:

The "very substantial withdrawal" Petraeus outlined Monday would keep the buildup in place for as long as possible without extending the tours of soldiers beyond the current limit of 15 months. Taking into account those tour limits, which were increased to their current level earlier this year, those additional troops would have had to come home anyway by the end of August. In essence, Petraeus was arguing Monday for a continuation of the buildup until virtually no more Army and Marine units were available.

More like gradual exhaustion, perhaps.