This is one of the hardest questions to advise upon when drafting contracts of employment for employer clients (or advising employee clients whether their covenants are enforceable). It depends what is "reasonable" and cases are of limited use, as each turns on its own facts. In other words, you have to second guess what a judge might think.
Here's an example:
"13.2. You shall not without the prior written consent of the Company directly or indirectly, either alone or jointly with or on behalf of any third party and whether as principal, manager, employee, contractor, consultant, agent or otherwise howsoever at any time within the period of six months from the Termination Date:
[...]
13.2.3 directly or indirectly engage or be concerned or interested in any business carried on in competition with any of the businesses of the Company or any Group Company which were carried on at the Termination Date or during the period of twelve months prior to that date and with which you were materially concerned during such period;"
So, a non-compete clause for 6 months and apparently with no territorial limitation. This is taken from the recent case of Egon Zehnder Ltd v Tillman [2017] EWHC 1278 (Ch) and was for a fairly high-powered executive headhunter in the financial services sector (a former European managing director and COO of JP Morgan before taking a career break). The reasonableness of restrictive covenants has to be assessed at the date the contract was entered into (rather than when the employee leaves), and here she had started at the most junior "consultant" level (albeit on a guaranteed minimum of £220k p.a.) but eventually left at the most senior "partner" level. The Group operated worldwide.
On first sight one might think 6 months is fine but no territorial limitation must be unreasonable. But that's not quite how Mr Justice Mann approached it in the High Court, which shows how difficult these cases can be to advise upon.
The defendant did indeed try to argue the covenant was unenforceable due to its global reach, but didn't get very far. The judge construed the restriction as only applying to businesses which competed with the businesses of Group Companies with which the employee had been materially concerned (and she had been concerned with some in other countries). Thus there was "an in-built restriction on the global reach to the clause, deriving from the need for Mrs Tillman to have been involved locally." This was therefore limited to what was reasonable to protect the Group's business. Worth bearing in mind when drafting non-compete clauses for clients who say their business is global so they don't want a territorial restriction. In reality they are unlikely to be operating in every part of the world, and involving the employee in those operations, so this formulation is a neat way of dealing with this.
The defendant also sought to argue that the clause prevented her from holding any shares in a competitor as an investment, and so went beyond what was reasonable to protect the employer's interests. However there was another clause that expressly allowed shareholdings of up to 5% as an investment whilst still employed, so the Court held that the non-compete clause couldn't have been intended to have that effect after employment. This shows the current approach of construing the wording to find the presumed intention of the parties, rather than construing any ambiguity against the party seeking to rely on the restrictive covenant. It is also a reminder it's best to include an express exception for investments of this sort.
On the reasonableness of the 6 months, the discussion centered on how senior this employee really was. The conclusion was that although she started at the junior consultant grade, she was clearly a high flyer whom the parties expected to move up the ranks (as proved to be the case). So although her contract had never been updated, she was sufficiently senior that the covenant was enforceable against her. "Six months seems to me to be a reasonable period," said the Judge, "principally to allow the substitution of new relationships with the client and for the fading of confidentiality." The implication here is that it could be tricky to argue for more than 6 months, even in fairly senior roles - though as ever it all depends on the facts of the particular case.
