As one who does this, please do know it is part of estate planning for us, but we don't attach strings to the gifts. If it is spent on a vacation, downpayment or paying off a car, that's great, too.
If you are honestly worried that she may run out of funds before she runs out of heartbeats, put it in a separate account or investment, still in your name but to be used if she needs it later.
It is given with love and hoping that you can enjoy when you want to and there if you need it. It also means that you are trusted with that amount of money - if you had a drug or gambling problem, it probably wouldn't be given directly to you.
I would be absolutely crushed if it were returned to us. As one receiver said, looking surprised, "Well, thanks. I can find a place for this." I don't know what he did with it, but it's fine.
I love "better given with warm hands than cold."