State laws and insurance companies' guidelines differ so there is no clear cut answer. Your MVR lists a violation date and a conviction date, most use the conviction date. In fact, we don't know of any company currently using violation date as that would be inaccurate.
There are some states however that do place offenses on your motor vehicle record (MVR) by the violation date instead of the conviction date, in those states the insurance company may choose to also go by the violation date but may also choose to go by the conviction date since that is when the violation will first be placed on your driving record by your DMV.
It is doubtful that your rates would change mid-policy or mid-term, either to go up after a violation or down when it leaves your driving record. Auto insurance companies typically do not adjust, uprate, raise or lower rates mid-term. For example if you have a clean driving record when you start the policy you are rated as such. If mid-term or mid-policy you get a speeding ticket conviction, your rates normally are not adjusted until the policy renewal period.
Usually an insurance company will pull your MVR at renewal time, at 36 months, or at the inception of a policy and thus if you got a ticket in January and were convicted in April then likely your rates were not affected by this moving violation until after April when your policy renewed. Thus it will likely be the renewal period after the offense leaves your record, or if your insurance company only rates on that offense for 3 years no matter how long it is on your MVR, that your rates will be lowered.
Here are some examples of when moving violations are placed on and taken off of your driving record in various states:
According to the New York State Department of Vehicles site, a moving violation conviction or an accident in NY normally remains on a driver's record during the year of the conviction occurred and for the following 3 calendar years. For this the DMV uses the year that the conviction happened, not the year of that the violation occurred.
The NYS DMV removes a conviction or an accident from a NYS driver's record on January 1 of the forth year after the conviction. Unless it is a serious conviction or accident of a serious type, those convictions can remain on a NYS driving history for more than 10 years.
In Illinois the IL Secretary of State (SOS) states that moving violations such as speeding, disregarding a traffic control light, improper lane usage, etc. remain on a driver's record for four to five years from the date of conviction.
Wisconsin DOT notes that convictions remain on the driver records for five years from the date of conviction.
California is different than many states because traffic offense convictions will remain on your record for a number of years from the violation date. For example most minor traffic violations will remain on a CA driver's record for 3 years from the violation date and count as 1 point. Accidents are reported for 3 years from the accident date.
To get information on if your violation was placed on your driving record by its conviction date or violation date contact your state's Department of Motor Vehicles.