Generally, you have to file Form I-751, Petition to Remove Conditions on Residence, within the 90 days period before your 2-years conditional green card expires (the exact expiration date is indicated on your green card).
There are certain cases when late filing is possible, for example, if you are applying for a waiver of the joint filing requirement. However, if you fail to file Form I-751 in due time, you risk losing your conditional permanent resident status. So it is better to meet the I-751 deadline and apply for removal of conditions within the mentioned 3 months period. And if you have missed the deadline consider consulting an immigration attorney.