This is an interesting question, and like most similar queries in history there's no incontrovertible answer. Only Henry could tell us for sure, but I'm not inclined to try to contact him by planchette, and he'd probably lie anyway.
Nigel Saul argues that he made the decision quite late in the process, indeed towards the end of August 1399. Ian Mortimer in his near-hagiography of Henry, Fears of Henry IV is confident that Henry had decided to take the throne before leaving France. (That reminds me, that book is still missing! Can the mice have eaten it? Is Henry's spirit playing tricks on me? Nah, he wouldn't have that much imagination or sense of humour.)
I incline more towards Saul's opinion, and that's the line I more or less followed in Within the Fetterlock. It doesn't mean I'm right of course, but here's my reasoning.
1. Henry subsequently quarrelled with his former ally, the Duke of Orleans over the matter. Clearly Orleans, who supported Henry's invasion, had been given the idea that Henry's ambitions stopped short of taking the throne. Orleans was outraged by Henry's accession.
2. Henry made a very public vow at Doncaster that he was not going to take the throne.
Now, you may say, Henry may just have been lying through his teeth all along. And you may be right. After all, if England had been selecting an Olympic Lying Team in 1399, Bolingbroke would have at least made the heats. But I have a feeling he was being honest as he saw it at the time. He didn't have a plan, but responded pragmatically to circumstances as they unfolded before him. (Rather as Richard III did in 1483, but that's a tale for another day.)
I think what changed his mind were the following factors:-
1. He was utterly amazed by the ease with which England dropped into his hands. This gave him an exaggerated impression of his own popularity. (He was soon to be cruelly disillusioned.)
2. The likes of Archbishop Arundel, who hated Richard II with a passion, were persuading him to take the throne.
3. He gradually realised (partly by being persuaded by the likes of the good Archbishop) that the only way to secure his own safety was to take the throne. Richard had a track record of making come-backs, and another of taking revenge. If Richard had somehow managed to regain power Henry would have been a dead man walking, and he wouldn't have been walking for very long.