The simple answer to that would be: Depends on how much money you're prepared to spend.
There are a lot of variables, although right now one of the most difficult problems in Portugal would be parking space in the airport!
Depending on size of the aircraft you would charter, it could be ready in a few hours, if you wan't anything bigger that a regular turboprop, you may have to wait until next day. Arranging a charter for the next day is very expensive. Arranging it a couple of weeks ahead saves you a lot of money, even cheaper a couple of months ahead. I have never asked for a quote for a "right now charter", but I have found operators willing to operate for some of my companys customers, when aircraft have gone tech. Some years ago, a Turkish GTI A300 was enroute to scandinavia within one hour after the request was made. That impressed me.
Some airlines specialize in adhoc charters and have standby aircraft and crew for those kind of things, I rememeber a Portugese airline with a slogan like "anytime, anywhere". They operate L1101s, so they should have space enough. Otherwise try calling Flightline, Titan, Falcon, Danish Airtransport, GTI (do they still exist), Blueline or Denim. There are many others, but I only remember these of the top of my head.
/FS