As long as your contractual notice period. Garden leave can only run during a notice period, so the maximum is whatever your notice clause specifies (commonly one to three months for most office roles, six months or more for senior roles, occasionally twelve months for the most senior executive contracts). Garden leave can also be shorter than the notice period if the employer brings you back or releases you early.
Garden leave isn't a separate period of employment with its own duration. It's a way of using up the notice period, where the employer keeps paying you but tells you not to come to work. So the maximum length of garden leave is the maximum length of your notice period. There's no statutory cap on it because there's no statutory cap on notice; the contract sets the figure.
For most office roles, garden leave maps directly to a one-to-three-month notice. People in those roles spend the notice working out, on garden leave, or on PILON; the figures are the same in all three cases. The employer chooses (or contract allows them to choose) which mode to use. Garden leave is more common at the senior end of that range; one-month garden leaves at the junior end are unusual because the practical value to the employer is limited.
Senior and executive roles have longer notice periods, and so longer garden leaves are possible. Three-month garden leaves for senior management are common; six-month garden leaves for director-level roles are not unusual. Twelve-month garden leaves do exist, usually for C-suite or highly competitive roles in financial services where the employer wants to protect commercial information for as long as the contract allows.
There's no minimum length either. An employer can put you on garden leave for a single day, for a week, for the last fortnight of a one-month notice, or for any portion of the period. They don't have to use the whole notice as garden leave. A typical pattern is to keep you in the office for a couple of weeks to do a handover, then garden-leave you for the remainder.
Garden leave can end early in three ways. The employer can call you back to work (legal but unusual; once they've put you on garden leave they usually keep you there). The employer can pay PILON for the remainder and terminate immediately (clean ending). You and the employer can agree to release you early without PILON (rare; the employer is giving up paid leverage). The most common ending is just letting the period run out.
Restrictive covenants change the effective duration significantly. Non-compete clauses typically start from the end of employment, not the start of garden leave. So a three-month garden leave followed by a six-month non-compete keeps you out of the market for nine months total. If you're moving to a competitor and the new employer is anchored to a specific start date, the date arithmetic here is the most important calculation to do.
Whether the courts will give credit for time spent on garden leave when assessing the reasonableness of a non-compete varies. Some judgments treat garden leave as serving the same protective function as a non-compete and treat the periods as overlapping. Others treat them as cumulative. The default assumption when planning your departure is that they stack rather than overlap. The garden leave calculator on this site shows both numbers.
Practical recommendation: read both your garden leave clause and your post-termination restriction clause together. They interact. If your contract gives a long garden leave and a long non-compete and you're moving to a competitor, model the total time-out-of-market before you accept the new role. Negotiate the new start date around the realistic clear date, not the resignation date.
General information about UK employment law. For your specific situation, contact ACAS or an employment-law solicitor.