15 or 20 minutes is pretty good. All I can say is that the more you practice a skill, the better you get. Define what goal you are shooting for - such as meditating for 60 minutes - and try to do a minute longer each day. Don't stress over it - just do your best and try to do a little bit longer each day.
Why is it so hard for me to sit still during meditation? I can only meditate up to 10-20 minutes until I start feeling like I need to stand up again.
When I was very young I was taught,
to meditate just before going to sleep.
My attention span at that time was compromised,
due to my living situation. The sounds of alternating
current or the ocean may help a great deal.