There is no restriction on the length of time a director has in paying a loan back to the company , its entirely a matter between director and the company .
It does attract a tax liablity if the loan extends beyond the companies financial year , once the loan is repayed the tax liablity will be repaid --
Its also possible the director could incur a tax liability if the loan is given at a preferential rate if the loan extends over 12 months , the liablity would be incured on the % difference on the interest between the preferential rate and general commercial rates
This may not apply in all cases dependant on what the loan was for
Any loan a director takes must not be so great that it affects the stablity of the company
A decent lawyer and accountant will be able to manipulate the company structure and finances to get around most restrictions without to much trouble