Joan, you will know when it is time, no one can tell you. When my mom passed she and my dad had made plans for her ashes to be put in with him when he passed away. My dad lived with me for about a year after she passed I had made a special place in a room just for her ashes, well my dad remarried and his new wife did not want my mom ashes so my sister and I talked about what my mom would want we thought about spreading them but my parents traveled alot and could not decide where her most favorite place was Alaska well I just could not do it, it took about another year for me to let go of her ashes but I was going off the deep end. I then decided to see if I could bury her with my grandparents also her parents,seeing you have to have permission to do this from all family members. Well I visit her knowing she would be ok with any decission I had made. My Dad Passed away Feb. 16,2009 and he is with her now. So take as much time as you need.
Joan, regarding the scattering of your mother's ashes... My mother took that responsibility out of my hands by requesting in advance that her ashes be scattered in the San Francisco Bay following cremation. Had she not written that in stone so to speak, I would most likely have found myself in your position.
Regarding your mother's ashes, when it feels right to you, do it and don't give the opinions of others any thought. This is a very personal thing and no one can take that from you, nor should they try. When people suggest "it's about time" just answer with the truth, that it isn't "time" for you and that though you appreciate their opinions, it is a very personal thing and you will do it when it's right for YOU.
I have friends that still have their parents ashes after 10 years and also have friends that scattered their parents ashes within weeks. BOTH are what was/is right for them.
Follow your heart and in the meantime, ask that you no longer be given advice regarding your mother's ashes.
I am so sorry for your loss and hope you find peace in knowing that your mother understands you holding on to what physical part of her that you have left.
One more thing, The Neptune Society has urns from thimble size on up, if letting go of your mother's ashes is too hard for you, maybe you could buy something small to keep your mother close to you and scatter the rest of her ashes. (laces like Marshalls, Ross and craft stores also sell appropriate "containers" for far less.)