In the absence of a codified constitution, one of the primary records of conventions and normal practices is Erskine May’s treatise on the law, privileges, proceedings and usage of Parliament
Paragraph 30.36 states the view that once a bill has completed all of its stages through Parliament, Royal Assent, must be granted - though it does not specify when:
When bills, either public or private, or Church of England Measures [...], have been finally agreed to by both Houses, they await only the Royal Assent to be declared to Parliament to give them, as Lord Chief Justice Hale says, ‘the complement and perfection of a law’, and assent must be forthcoming.
It is common, when multiple bills all complete their parliamentary stages close together in time, to receive Royal Assent in one go, and a short delay to facilitate that is uncontroversial.
Aside from that, though, there is an implication that there should not be any undue delay, as in the case where a bill completes its parliamentary stages earlier than expected:
Care is taken that all bills which are expected to have passed are included in the Letters Patent. Nevertheless, a bill not named in the Letters Patent may unexpectedly pass during the period after the submission to the Sovereign and before the Royal Assent is declared to Parliament. Since there is no authority for withholding from inclusion in the Letters Patent any bill which has passed all its stages, a further submission by the Lord Chancellor to the Sovereign must then be made for the Royal Assent to be given to the additional bill and for its inclusion in the Letters Patent.
This also appears to address the separate but related question as to whether the government can either advise the Queen not to give her assent to a bill, or decline to present it for assent at all, the answer being 'no' in both cases.
It should be noted that Erskine May is not law; but convention carries significant weight in the British system, and breaching it is not without political consequences.
