I bought myself a highly inexpensive holiday watch at the beginning of July. A Casio, the one on the left below. I set it to the precise time on the evening of July 4th and was surprised a few days later to see that it hadn't apparently budged.
35 days later it seems to have gained about 1/3 of a second.
I'm quite surprised by this, given that it's available new for a bit less than £7 from Amazon. By contrast the Seiko solar PADI diver that I bought at about the same time has gained about 3 seconds, which is more than acceptable of course.
So I've just bought another one to see how that does (pictured on the right). I'll let you know. I have applied a subtle distinguishing mark to the rear of the keeper, with a silver permanent marker.