How long - The reference to Exodus 16:4 is obvious. The prohibition involved a trial of faith, in which as usual the people were found wanting. Every miracle formed some part, so to speak, of an educational process.

And the Lord said unto Moses,.... Who had seen and taken notice of what those men had done, who went out into the field to seek for manna on the seventh day, and was displeased with it, and therefore spoke to Moses out of the cloud:
how long refuse ye to keep my commandments and my laws? this is not said merely with respect to their breach of the commandment of the sabbath, as if they had long refused to observe and keep that; whereas that was but one command, and but just given; but upon their breach of that, he takes occasion to upbraid them with their former transgressions of other laws of his, and which they had continued in, or at least were frequently committing; and which was a proof of their perverseness and rebellion against him, though he was so kind and bountiful to them.