From what I've experienced, if you don't actually use your shield (guard, aegis boon, shield bash, etc), it doesn't really seem to wear out much. For example, I use a 1-h weapon and have a shield equipped for an evening of killing stuff, and by the end of the night my weapon has worn down considerably but my shield has not because I don't use it. I ranked sentinel up to 50 while playing DoM classes but the only time I can recall needing to get a shield fixed was during the time where I did mostly soloing and actually used my shield skills a lot. And when I (briefly) played GLA I also noticed wear 'n tear on my shield.

That said, I went by sentinel rank, not the rank of the main class I was playing, as that was the advice I was given. I don't think anyone really knows for 100% sure though! (and if they do I hope they will come post here, I am curious too lol)