How soon does the PM have to present a passed bill for royal assent?

Question

How long can the UK government delay presenting an otherwise passed bill to the Queen for royal assent (could they perhaps delay it indefinitely to prevent it becoming law?).

Parliamentary pages I've read say 5-10 days is normal & iirc more than 15 highly unusual, but, I've not found anything definitive on how long the government can take to present it or even if they have to present a bill for royal assent at all if they wanted to frustrate it's passing into law.

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Answer 1

On the Today programme on BBC Radio 4 on 02/09/2019, David Gauke claimed that by constitutional convention Parliament could not be prorogued while legislation was waiting for royal assent.

So in the specific instance I imagine you are probably referring to, with the Boris government delaying legislation to prevent a no-deal brexit, the answer appears to be "until they want to start prorogation."

This is yet another example where there is no written rule, it's simply the UK's unwritten constitution which consists of a set of conventions, so the general answer depends on the government's will to cause a constitutional crisis/outrage and the possible use of court action to force its hand.

Answer 2

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.