Probably not, there was no set term mentioned in the paperwork I had but I was asked if Mum had given any assets away in the last 20 years.
As soon as she went into care she would be exempt from Council Tax on the house, and from what you say she would qualify for the 12 week house disregard which means social services would pay, but would still take her pensions into account. I really sympathise with you having gone through this twice with both my mother and mother in law. I got more advice with my Mum as like yours she had no assets apart from the house, because my mother in law had savings, social services did not want to know at all.
Options open to you could be to defer selling the house, a charge will be taken against the property and all the money claimed when she sadly dies.
Sell the property and purchase an annuity which will pay the fees and invest the rest.
The NHFA I mentioned earlier is the Nursing Home Financial Advisors and is part of HSBC, you may find a fact finding meeting with them helpful just to think through your options. It would be free but don't forget that at the end of the day they would be looking for your business. They are Independent Advisers by the way.
Hindsight is a wonderful thing (and I am making assumptions here) but if your parents owned the house as tennants in common and your dad had left you his half in his Will I am pretty sure they could only have claimed your Mums half.