When should I Bottle

My airlock is bubbling about once every 15-20 seconds and has been fermenting
for 10 days. I've read that you are supposed to take hydrometer reading to make 100% sure it is done fermenting, but how can I tell from the frequency of its bubbling? Thanks

You can't. The best way to be sure is to look for the gravity readings to be the same over the course of about three days. Otherwise you can just leave it for three to four weeks and it will most likely be done by then.

Hydrometer reading only. Don't risk bottle bombs man, not worth it.

What those guys said.

As above stated, the only way you'll know it's at FG is if you have a pair of matching readings 3+ days apart. OR, just give it enough time to finish, clean up after itself, and then let you know it's ready for next steps.

Brew more batches, try using as few different yeast strains as possible so you can learn how they work at the fermenting temperatures. IMO, this is one of the best things you can do for your brewing. You'll find out which yeasts produce what flavors at what temperature ranges and such. I've been doing this for some time now, using the same few strains of yeast for my brews. I also know what temperature my brew is fermenting at (thermowell with a sensor down it). I give the batches enough time to become great and be ready to go to keg. I don't fuss with them, take extra readings, etc. I simply KNOW that the yeast will give me what I want as long as I've given it the time it needs (dictated by OG, pitching a proper yeast colony size, and temperatures).

I suggest reading up about making yeast starters (especially for liquid yeast strains) and fermentation temperature control. Those two items will do more for your batches than a lot of other things.

I don't even bother for a reading until at least 12 days, mostly 14. Like everyone else says: stable gravity for a couple days is he best way. It's never good to bottle based off airlock activity.

I also agree that hydrometer readings are the only true indicator. Also, I very much agree with Golddiggie about letting yeast " clean up after itself." I leave all my ales in primary a minimum of 3 weeks, but most average 4-5 weeks. It's amazing how clean the brew gets and I have no need to cold crash. Plus, it can't hurt to wait. My friend, who loves citra hop beers, put hours of time into planning and brewing his ideal hoppy imperial pale ale. He then spent even more time waiting for primary, secondary/dry hopping, and keg carbing to complete. He was so concerned with losing hop flavor/aroma by leaving it in primary too long, that he rushed the primary into a secondary/dry hop to soon. Weeks later.....diacetyl monster. Moral of the story: just wait! Admittedly, it's easy to say for me because I can brew multiple batches at a time. If you are a 1 brew at a time brewer, I understand the desire to push through as quick as possible, but you may end up with butter popcorn ale.