Juniper uses the concept of certification status for either the recertification at the same level or the advancing of the certification to the next level.
When you pass the professional level exam you will have active certification status for two years starting from the exam pass date. For the third year your certification status is inactive. After this the cerfification status is expired.
You are able to advance or recertify from professional to expert during either the active or inactive status. You cannot advance or recertifiy when the certification is expired.
Thus you have three years from the date of your pass on the professional exam to sit for the expert lab exam.
See the certification status page for fuller details.
http://www.juniper.net/us/en/training/certification/already-certified/recertification/