I can't stress enough how many North Africans we are talking about here. If you are travelling from Algeciras to Morocco betweeen 15th June and 15th September, this is when the official Paso del Estrecho operation takes place, where hundreds of civil protection teams, police and red cross workers all work to get a million families
to and from their homes in France, Belgium, Holland and Italy, with most of them crossing the Straits of Gibraltar at Algeciras.
Having said that, the vast majority of vehicles travel in July and August, with waits of up to 2 days at the port not uncommon. You might get an idea from this page I found, with a very rough translation from Google! http://translate.google.com/translate?hl=en&sl=es&u=http://www.el-mundo.es/1998/08/01/madrid/&prev=/search%3Fq%3D%2522operacion%2Bpaso%2Bdel%2Bestrecho%2522%26hl%3Den%26safe%3Doff