Asking if a specific deficit is safe is hard to say - someone could have a strict diet at 600 calories and be OK while someone else is less strict at 900 calories a day and have issues. People worry about going too low on calories. Most of the time you don't lose muscle/health from a caloric deficit, you lose it from a protein deficit. If you get sufficient protein, vitamins and minerals - your caloric intake does not matter.
Having said that, John.Patterson's advice is pretty easy to agree with.
One thing I will say - if you're looking to create a deficit you are usually better to do it through reducing dietary intake than through increasing exercise levels. Too much exercise increases your caloric needs, protein needs, recovery needs, etc. while reducing leptin levels and causing more hunger. I'm not afraid of someone with a 2100 cal/day TDEE going to 1000 calories of food intake a day (provided you get at least 0.8 g protein/pound of lean mass) - but I would warn against some pitfalls of going to 1500 calories a day and doing 1500 calories in exercise to boot.
I am typing this message after having spent 6 months last year eating 600-800 cals/day with 1-2 cheat meals/week. It isn't recommended and if you're going to go into more than 1,000 calories/day deficit I really recommend you read some of Lyle McDonald's work.