I drove it for about 3 weeks while it was like this, just kept topping up the coolant level and kept an eye on the temp gauge, but it will get worse the more u drive it as the crack in the core will get bigger and bigger, also put a towel or something in the footwell to try and avoid the damp carpet rusting the car. (i was going through about 1 liter of coolant a day towards the end)
i picked up a 2nd hand core for $60 bucks followed this tute and now have a problem no more.
Having said that it is easy to Bypass the heater core for the time been and there is easy DIY on how to do that so coolent will no longer get to the heater core, like this u can drive it for 20 years you just wont have a heater.