Hacking a system takes one full action per tier. If you fail, countermeasures activate. However, I don't see anything in the rules about when they activate.
Do they activate in the first round of that task? If so, you'll immediately know you've failed and can try again, only having wasted one round.
Or do they activate only on the last round of the task? That would mean that you have to do the whole task before even checking if you've failed.
The latter would be extremely unforgiving, given how hacking already takes forever. It would be in line with other multi-round tasks like Disable Device, though. You have to do the whole task, even if you fail. However, with Disable Device, you don't even know you've failed which is an important difference to hacking.
Not sure which is correct, but I would probably run it as "at the end" unless there is evidence for the contrary.