Previously prisoners could be considered for parole after serving just seven years.
But the change comes with Justice Minister Heather Humphrey's establishment of a parole board and the commencement of the Parole Act.
"I think it could be a lot longer, especially if they get a life sentence," Rose Callaly told the Herald.
"As long as we have breath in our body we will keep fighting for fairer sentences, for the family of victims.
"I'll keep supporting any move that's made to increase the mandatory sentence. It's maybe moving in the right direction, but it's not there at the moment."
Ms Callaly's daughter Rachel was bludgeoned to death by her husband Joe O'Reilly in Naul, north Co Dublin, in October 2004.
Since then O'Reilly has applied for parole on a number of occasions, although he remains in prison.
Rose has campaigned for stricter sentencing, especially for murderers.
While she welcomed the 12-year limit as some progress, she said it should be at least doubled.
"Then we might support it. At least it's moving in the right direction," she said.
"I think it could be a lot longer - especially if they get a life sentence. We were hoping it would be a longer time.
"Even a life sentence in this country, people serve 20, 25 years. It's not long enough.
"Our Rachel hadn't even had time to reach her potential and he decided to take her life," she said.
"Then he was able to apply for parole after a few years. It's bizarre."
She said even under a 12-year limit, O'Reilly can still be considered for parole. He was sentenced to life in 2004.
"He'll keep on trying. It won't make much difference to him.
"He hasn't served as much as he deserves. It's bizarre the system we have," she said.
"I'll keep doing anything I can to make things better. It might not benefit us, but if it benefits others..."
The Department of Justice has said the purpose of the new act is to place the parole process on a statutory footing and establish an independent, statutory parole board to decide on parole applications.
Mr Justice Michael White will be the chairperson of the new statutory parole board after being nominated by the Chief Justice.
A further nine board members were appointed following their nomination by specified nominating bodies and post-holders.
Parole will only be granted to prisoners if the board is satisfied that they do not pose an undue risk to the public, that they have been rehabilitated and that it is appropriate in all the circumstances to release them on parole.
The average sentence served in the last 10 years before a life-sentenced prisoner is released on parole is 18 years.