Though the ROP website doesn't state any wait period, many of the companies tell there employees about 6 month wait period. Based on the position and the company, it can be anytime after your spouse has received his resident card.
There has been divergent views on required documents too. Hence it is better to seek advise from the sponsor company or its authorised PRO.
For the benefit of all let me reproduce what is stated in ROP website :
FAMILY JOINING & FAMILY RESIDENCE VISA
The family Joining Visa is granted to the wife of the foreigner, who is resident in Oman, and as well to his children who are below (21) years. It is also granted to the foreign wife of an Omani national at his request, and subject to a certificate from the concerned authority (Ministry of Interior) confirming the status of the marriage.
The family Visa is granted by the concerned authority at its discretion, and at the request of a local sponsor and on his responsibility, to the relatives of the Omani national and the relatives of the foreigner who are not included in the categories eligible for Family Joining Visa. The Visa shall be as follows:
Valid for use: Within six months from the date of issuance.
Validity: Two years from the date of stamping the visa in the passport.
Fees: 20 Omani Riyals.
Fines for late renewal or non-registration of the residence permit is (50) fifty Omani Riyals per month.
Conditions:
The sponsor must be a local one with full legal capacity, and he is held responsible for the truth of the information stated in the application form.
For the employee's children: Their age must not be above 21 calendar years.
(Note: Their residence permit expires when they reach this age)
Brothers and sisters must not be above 18 years. A document confirming that they have no family provider outside Oman must be presented.
The family visa expires when the employee's residence permit expires.
Required enclosures:
Submitting the electronic application form available through the website, to be endorsed and stamped by the sponsor, and to be typed in Arabic for Arab nationals and in English for Non-Arab nationals, enclosing the following documents:
Two photographs size (4X6) cm.
Copy of the foreigner's passport, provided it is valid for a period not less than six months.
Copy of the passport of the foreign resident who wants his family to join him, and a copy of his residence card.
The standard undertaking form specified for the family joining visa applications. The undertaking form must endorsed by the sponsor and his foreign resident employee. Exemptions apply for those below (15) fifteen years.
A proof of the family relationship must be enclosed, unless the names and passport details indicate the relationship between the foreign resident and the person who will join him.
For the spouse, an original conformed copy of the marriage certificate must be enclosed. The document must be endorsed by her country's embassy and the Omani Ministry of Foreign Affairs.
Copy of the specimen signature of the authorised signatories issued by the Ministry of Commerce and Industry, as for the personal sponsorships, a copy of the personal identity card must be enclosed for signature verification.
In case the application is presented by someone other than the sponsor, the representative must be Omani and holding a written authorisation issued from the Directorate General of Passport & Residence as per the sponsor's request.
A medical fitness certificate issued for citizens of some countries specified as follows by the Ministry of Health (India, Pakistan, Philippines, Bangladesh, Indonesia, Sri Lanka, Egypt, Syria, Sudan, Ethiopia); certificate must be endorsed by the Ministry of Health.
If the visa application is requested for the foreign wife of an Omani national, a certificate from the Ministry of Interior confirming the status of the marriage must be presented.
Best Wishes