Hello Meenu,
There is no hard and fast rule for notice period and it depends on what is agreed between the employer and employee under the contract of employment. As you have stated that its a senior person, in such cases normally there is no probation at all, since the person at such level is hired after much research and track record. In-spite of it if the company wants to have a probation period, it is advisable to be limited at 6 months, but most importantly you also need to have a reciprocal clause for immediate severance just in case. Since employees at such senior level either stay for long or if it is not their cup of tea, which is rear though, they leave within weeks. Once such employee crosses even 3-5 months you can safely predict that he will stay for good. Once permanent I suggest 3-6-9 pattern of notice period, which means the employee can choose to give either 3, 6 or 9 months notice depending on his needs, like wise the company can compensate his absence by deducting notice pay of 3 months if he gives notice of 3 months, and not deduct any notice pay if notice is for 6 months. 9 months is very unlikely unless it is mutually agreed between the employee and employer in case a suitable replacement is not available, similarly the employee also can refuse if he has opportunities and cannot wait for more than 3 or 6 months. Senior positions are tricky to handle some times so co-ordinate well with his reporting boss and the employee while framing the clauses in the contract.