I am following the Alfred's adult course level 1 book and given that songs are very simple I have come across an issue when exactly do I know I'm ready to move on to the next lesson? I tend to go if i play this piece three times in a row with no error I'm ready is this a good rule? Because by no shape or form have I mastered these songs it just seems a bit odd to spend days or even weeks on something a simple as row your boat if I get the gist of the song and if my successful plays out weight the unsuccessful ones I go the next lesson is this wrong? When did you ladies and gents knew it was time to move on? I'm lost.
I think it's best to always play a little above your level so you are being challenged. First of all you don't want to get bored.
Secondly being perfect is a goal that will just drive you crazy. Learning an instrument is not a linear curve. I look at it as more of a two steps forward and one step back. And by that I mean move on to a harder piece, but remember that there will always be something more you can get out of a tune, even a simple one like row row. Keep pushing ahead unless you are really struggling and then by all means slow down and work on stuff.
Also there have been studies about leaning that show we do better if you practice multiple things in a session and move around a bit. So maybe have three or four things you are working on in a practice session and divide your practice up between then. Source for that: https://bulletproofmusician.com/why-the-progress-in-the-practice-room-seems-to-disappear-overnight/
This link also has good stuff about, limiting distractions, setting goals, and other general practice tips: https://www.npr.org/sections/deceptivecadence/2013/09/03/216906386/10-easy-ways-to-optimize-your-music-practice