How long do you feel you need to meditate for it to be effective?

In terms of time, I find each day is different. Sometimes I have the time for a longer meditation-10-15 minutes. Sometimes I feel I need to meditate for that period and I feel amazing afterwards. On other days I feel the need for a shorter burst-almost an energizing burst.

It takes a different amount of time for everyone. If you go into meditation with expectations of rapid results…you arent allowing yourself to be open to the peace that eventually follows.

Studies vary but I have found 5-7 mins daily for a month has made an appreciable difference to my mental strength and flexibility. When paired with my gratitude ritual before bed it’s very powerful.

As long as you meditate every single day with out fail, wether it's 60 minutes or 3 minutes, that's all that matters. For me the most effective is 10 minutes, but if I don't have time I'll do it for 3

Around 10min a day; I find this a good time to Integrate meditation in my daily practice, long enough to bring my mind back when it drifts away, but short enough to leave time for other actions which are part of my routine.

I do 5 minutes in the morning and 10 minutes in the evening. But i feel im just scratching the surface

If you mean how long in one session, it's not really about how long, but about how well I can empty my mind and focus, bringing back my attention whenever my mind would wonder.
If you mean in general, I usually start to feel the change in my mental state during the day already after the first time.
For a lasting change I have to build these changes into my personality and for that I think around one month would be ideal.

Ideally 15 to 20 minutes in the morning and for rest of the day it's 10 minutes towards each session

I have been doing 20-minute sessions a day for a while now and that's good to feel effective for the day for me. But even when I was first starting out I would feel calmer after only 5 or 10 min.

I use the Calm app, the free version and I play and replay the guided meditation sessions. So far I'm at day 16. I feel so much more relaxed than I did two weeks ago. I intend to keep meditating as part of my ongoing mindfulness practice.