The status of employment, i.e., temporary or permanent, depends on two factors: the tenure of the employee on the job/post and the nature of the job/post. You can temporarily employ a person on a job of permanent nature due to reasons like the leave of absence of the incumbent already holding that post. Certain jobs may be of limited duration, and you may rotate the services of a temporary employee continually among such posts. Therefore, in my opinion, what ultimately decides the status of an employee, whether permanent or temporary, is their length of continuous service in the same organization.
Continuous Service and Unfair Labor Practices
Section 25-B of the Industrial Disputes Act, 1947, defines continuous service as an uninterrupted stretch of service in the same establishment with certain permissible interruptions mentioned therein. Item no. 10 of Part I of Schedule V of the same Act defines that employing workmen as temporaries and continuing them as such for years, with the object of depriving them of the status and privileges of permanent workmen, is an unfair labor practice on the part of employers. So, one cannot precisely set the maximum tenure of temporary employment. This is only a general observation with reference to the limited inputs available about your new company.
Applicability of the Industrial Establishments (Standing Orders) Act, 1946
I don't know whether the Industrial Establishments (Standing Orders) Act, 1946, is applicable to your new company. In any case, however, a broad answer to your query based on the above Act can guide you properly to some extent. The Model Standing Orders under Schedule I appended to the Industrial Employment (Standing Orders) Central Rules, 1946, define the following classification of workmen as below:
MSO 2(b): A "permanent" workman is a workman who has been engaged on a permanent basis and includes any person who has satisfactorily completed a probationary period of 3 months in the same or another occupation in the industrial establishment, including breaks due to sickness, accident, leave, lockout, strike (not being illegal), or involuntary closure of the establishment.
MSO 2(e): A "temporary" workman is a workman who has been engaged for work which is of an essentially temporary nature likely to be finished within a limited period. Therefore, you have to decide for yourself which jobs in your company are permanent or temporary and how long you can keep an employee as temporary.
The above reply is in respect of those employees falling within the definition of "workman" as defined under section 2(s) of the ID Act, 1947, only. In respect of others, you can devise uniform norms postwise either in the contract of employment or in the service regulations to be made.