Intensive meditation (which might or might not get intense – the two words are not synonymous) would IMHO be anything upwards of eight hours of sitting per day. On Goenka retreats, you sit ten hours a day. In the Mahasi tradition, you switch between sitting and walking all day long, often totalling sixteen hours of meditation all in all.
If you are new to meditation and have not done retreats before, I recommend being careful. Some yogis (including myself) have pushed them selves too hard on retreat and fried them selves. It can do real damage. And it is very individual where your limit is, and not always easy to figure out beforehand. If you have trauma, especially (even partly) unresolved trauma, in your bagage, the risk of adverse effects is bigger.
So I would say, start by doing one whole day of meditation. For instance eight times one hour of sitting, interspersed with some yoga, tai chi, qi gong or other form of mindful movement, and with some periods of rest. Observe "noble silence", switch off all communications and devices. Do not distract yourself with anything that whole day. And if that goes okay, then maybe two days, then maybe five etc.
Best wishes!
