If an inventor has been left out from a patent application, which has been filed, is there a deadline for including the inventor in the patent?

I can only comment on US rules. Based on this posting, consult an IP Professional ASAP!!! Willfully leaving an inventor out, which you are admitting to HERE IS a bad thing.

http://www.legalteamusa.net/tacticalip/2013/06/10/improper-inventor-on-a-patent-application/

For Dutch and European patent applications the deadline for providing the details of the inventors is 16 months after filing or - in case priority is claimed - after the priority date. This means that at the moment of publication of the application it will be visible to third parties who have been mentioned as inventor.
Both for the European patent application and for a Dutch application the inventor the applicant (company) is obliged to file a declaration of inventorship (Art. 14 Rijksoctrooiwet, Art. 62 EPC)). However, in practice it is difficult to enforce this right to be mentioned Ultimately, one can address a court to ask for an order to enforce the applicant to submit a declaration of inventorship with the patent office, but as far as I know thai has not yet been done.

The situation that you deplore is not classic farmhouse anecdotal. To my knowledge and given what I have seen in relation to French, European and international (PCT) patent applications, it is not possible to return to the omission of a co-inventor in the application patent, at least once the latter published. If it is namely a Dutch patent application, perhaps the Netherlands rules permit you may remedy the situation. In all cases, the most appropriate would be that you consult a patent attorney in your country or directly the NL Patent and Trademark Office (Octrooiwet) to find out what you can do. If the administration refuses to make the addition requested, the injured co-inventor may certainly have the possibility to take legal action (action for recovery) but this action concerns normally fraudulent filings and not accidental ones...

I remain at your disposal should your inquiry relates to France or Europe.

Your sincerely,
Simon

FOR U.S. ONLY - See USPTO AIA Blog, Mon., Jan 07, 2013.

For example: For a Non-provisional patent application, Adding or deleting inventor(s) requires:
•A request under 37 CFR 1.48(a) to correct or change the inventorship;
•A signed ADS including inventor information for all actual inventors as required by 37 CFR 1.76(b)(1); and
•An executed inventor’s oath/declaration under 37 CFR 1.63 for any added inventors (37 CFR 1.48(b)).
Note: For applications filed prior to 9/16/12, where a 37 CFR 1.48(a) request is filed after 9/16/12, the inventor’s oath/declaration must be compliant with the prior version of 37 CFR 1.63, including the requirement to identify the entire inventive entity. However, only the added inventor needs to execute the oath/declaration identifying the entire inventive entity.
FEE: $130 per 37 CFR 1.17(i)