Refinement time is 10 % of DEV team capacity

In my team we spend, with 8 developers and an iteration length of 3 weeks, 1,5 hours a week on backlog refinement.

3 weeks = 15 days = (15 * 8) 120 hours per developer. 10% is 12 hours per sprint. You can hold the refinement at any time.
If you divide this over your 2 sessions, that would make 6 hours per session which is quite long.
Note that I assume 100% of the time towards working on backlog items. I guess you have some slack built in or other stuff that limits the capacity.

Why not take 2 hours per session and see if that is enough for your team. Over time you might cut down on the time needed to refine or scale up a bit (as you have plenty of refinement time left).

> The Scrum Guide suggests that "refinement usually consumes no more than
> 10% of the capacity of the Development Team."
>
> Can you explain if this is for one meeting or total refinement meeting in a Sprint ?

Product Backlog Refinement isn't a formal Scrum Event, and so it is best to view it as an ongoing activity rather than as a discrete meeting or series of meetings.

Refinement *might* be conducted as meetings, and each may be time-boxed just as if it *was* a Scrum Event. These meetings would typically form at least part of a team's refinement activity, and the formality of a Scrum Event would therefore be emulated to a greater or lesser degree. However, there are other possibilities. For example, refinement can include investigative "spikes" where developers prototype certain features in order to better understand them, or the technological choices involved, and so allow the probable work to be estimated more accurately.

1.I want to mention that Product backlog refinement is not time box even though the scrum guide specifically states that it should be 10% of the capacity of the development team.

2. Product backlog refinement is dependent on the development team capacity, and since the capacity of every team is different, PB refinement time will be different,  so its varies from team to team.

3. Most importantly time has to be made by the team for product backlog refinement every splint for a value added product. 

@Forson, a couple corrections to your recent statements:

1.I want to mention that Product backlog refinement is not time box even though the scrum guide specifically states that it should be 10% of the capacity of the development team.

The Scrum Guide does not explicitly state that refinement is 10% of a Development Team's capacity, nor does it state that it should not consume more than 10% of a team's capacity.

As stated in the Scrum guide and in the initial post of this thread:

"Refinement usually consumes no more than 10% of the capacity of the Development Team"

It could be more, it could be less, and there are other factors involved besides Development Team capacity.   Team maturity, item priority, and item complexity can also influence the amount of time a team spends in refinement during a sprint. 

The word usually is the keyword. Refinement can take as much time as the team needs to refine the backlog items for the upcoming Sprint. It could be more during the initial sprints while could slowly stabilize during later sprints. Again this is only could be. 

The Scrum Guide suggests that "refinement usually consumes no more than 10% of the capacity of the Development Team."

Can you explain if this is for one meeting or total refinement meeting in a Sprint ?

Simple, do you think the capacity is for entire sprint or only on the day you plan refinement? So it is for the team capacity of the entire Sprint. This will answer your next question

Say if we have 6 DEV in 3 week sprint , and we want to have only 2 refinement sessions , how much time we can spend ?

It does not matter whether you have 2, 3 or one sessions (I would suggest a more consistent and regular approach of once a week that will address the responding to change from Scrum Guide. All you have to try is to not to cross 10% of the capacity. Note: even this is not mandatory the word usually means its an observation and best practice and not mandatory ! you may have a complex product and may need more than 10% to refine. The work usually lets to cross this time. So do not try to play it to the book especially when the book uses the words usually for obvious reasons.

For a 40 hr week if you want to stay close to the suggestion you need to spend less than 10% of 40 hrs = 4 hrs of refinement sessions per week. Therefore it does even matter how long your sprint is! It's a simple understanding of not to cross 4 hrs (10%) of a weeks 40hrs time per individual (all of them together spend 4X6 = 24 man hours of course !)

In short ...

Just try to keep your refinement sessions under 4 hrs a week

....unless your product's complexity needs more time especially at the beginning or during technical spikes (-->NOT a scrum term!)

This statement "Refinement usually consumes 10%..." has been removed from the Scrum guide 2020