lord of the rings - Saruman in the Shire: how long had he been planning it?

Saruman had been positioning himself in the Shire for quite some time, at least a couple of years and potentially as long as a decade or two. This is foreshadowed during the fall of Isengard in the Two Towers:

‘We understand it all perfectly now,’ said Gimli.

‘All except one thing,’ said Aragorn: ‘leaf from the Southfarthing in Isengard. The more I consider it, the more curious I find it. I have never been in Isengard, but I have journeyed in this land, and I know well the empty countries that lie between Rohan and the Shire. Neither goods nor folk have passed that way for many a long year, not openly. Saruman had secret dealings with someone in the Shire, I guess. Wormtongues may be found in other houses than King Théoden’s. Was there a date on the barrels?’

‘Yes,’ said Pippin. ‘It was the 1417 crop, that is last year’s; no, the year before, of course, now: a good year.’

‘Ah well, whatever evil was afoot is over now, I hope; or else it is beyond our reach at present,’ said Aragorn. ‘Yet I think I shall mention it to Gandalf, small matter though it may seem among his great affairs.’

Saruman had had quite a long relationship with someone in the Shire, who supplied him with pipeweed.

We also have two dates which show Saruman becoming aware of the Shire having a special significance, from Appendix B:

2953 Last meeting of the White Council. They debate the Rings. Saruman feigns that he has discovered that the One Ring has passed down Anduin to the Sea. Saruman withdraws to Isengard, which he takes as his own, and fortifies it. Being jealous and afraid of Gandalf he sets spies to watch all his movements; and notes his interest in the Shire. He soon begins to keep agents in Bree and the Southfarthing.

(...)

c.3000 The shadow of Mordor lengthens. Saruman dares to use the palantír of Orthanc, but becomes ensnared by Sauron, who has the Ithil Stone. He becomes a traitor to the Council. His spies report that the Shire is being closely guarded by the Rangers.

Saruman has been aware of the Shire for about 60 years (and got reports from at least 17-18 years), and has been obtaining pipeweed from it for at least two. As a result you can conclude that Saruman did already have his webs of treason and corruption in the Shire well before the Hobbits even left the Shire. After all, they noted Big People (Men) causing trouble as they were leaving the Shire in the Fellowship of the Ring:

‘But what has one of the Big People got to do with us?’ said Pippin. ‘And what is he doing in this part of the world?’

‘There are some Men about,’ said Frodo. ‘Down in the Southfarthing they have had trouble with Big People, I believe. But I have never heard of anything like this rider. I wonder where he comes from.’

There's no explicit link between these Men and Saruman, but as you say, he would have had to have his heavies relatively close by and large in number to enforce his martial law. Not all of them would have come down from Bree.