Level Gaining

For those out there who have managed to get their leads up to and/or past level 15, about how long would you say this took you to do? I've been playing casually daily between other things (trying to make sure I do all my exploring efficiently) and I made it to level 10 two days ago...but haven't gained another level since. I'm just barely scraping 11 now but it's been a GRIND getting there. 

I'm not complaining of course since I figured it'd get harder- and I don't always have the best luck with battles swaying in my favor- but I was just wondering what other players' experiences with leveling have been? Do you have any tips on leveling more efficiently? I'm just so eager to get to search for those special BNWs but I still have a long ways to go yet I feel like.