registration can be done within maximum period of 8 months 

2) after 4 months registration can be done but with payment of penalty 

3) In case the time limit has expired, you can make an application to the sub-registrar for condonation of the delay, within the next four months and the registrar may agree to register such documents, on payment of a fine that may be up to ten times the original registration fee. 

Four months from the date of their execution, along with the requisite fee. In case the time limit has expired, you can make an application to the sub-registrar for condonation of the delay, within the next four months and the registrar may agree to register such documents, on payment of a fine.

Sir the stamp duty is not valid after 6 month if not put to use in Maharastra you can deposit it back with Collector and can receive refund of same.  

Yes the penalty can extend upto 10 times the registration amount.

it may be upto 10 times the original registration fees not necessarily 10 times 

if you are not sure about the registration then go for refund else pay late payment charges and do the same.

as per s.23 read with s.25 of the registration act, a document can be presented for registration within max period of 8 months from date of signing

if document is presented for registration within 4 months of signing, then it can be registered by the officer

however if the document is presented for registration after expiry of above period of 4 months and thereafter presented for registration within the next 4 months, then on paying of a fine, the document will be registered

the fine is upto 10 times of the registration fee

it is to be kept in mind that the max penalty is 10 times of registration fee

so the registrar may after considering the reasons given by the person for his inability to present the document for registration within the prescribed time, may levy a fine lesser than 10 times of registration fee by passing an order 

please read the text of s.25 as below:

Provision where delay in presentation is unavoidable
 
(1) If, owing to urgent necessity or unavoidable accident, any document executed, or copy of a decree or order made, in1[India] is not presented for registration till after the expiration of the time hereinbefore prescribed in that behalf, the Registrar, in cases where the delay in presentation does not exceed four months, may direct that, on payment of a fine not exceeding ten times the amount of the proper registration-fee, such document shall be accepted for registration.

(2) Any application for such direction may be lodged with a Sub-Registrar, who shall forthwith forward it to the Registrar to whom he is subordinate.

Where you paid stamp duty ?and registration pending at whose end ? After 4 months, grace period of another four months is allowed on payment of a penalty.
Yes.

"27. Scale of fines under section 25 or 34:-

The scale of fines to be imposed under section 25 or 34, shall be as follows, namely:-

(1) If the delay does not exceed one month-not exceeding 2 1/2 times the proper registration fee;

(2) if the delay exceeds one month but does not exceed two months-not exceeding five times the proper registration fee;

(3) if the delay exceeds two months but does not exceed three months-not exceeding 7 1/2 times the proper registration fee;

(4) if the delay exceeds three months but does not exceed four months-not exceeding ten times the proper registration fee.

Note: This rule does not affect the Registrar's discretion to impose a smaller fine than the above maximum, under sections 25 and 34, in suitable cases."

Akshay Vitta Management and Investment Consultancy Services Pvt. Ltd. and Ors. vs. State of Maharashtra and Ors. (21.04.2015 - BOMHC)

Why is the registration process pending, is it due to your fault or the registrar's fault?

As per Section 54 of the Indian Stamp Act, if you do not have any immediate use of a stamp paper, then you can deposit it back to the collector within six months from the date of purchase and take back your money as a refund after deduction of 10 paise per rupee.

In Thiruvengada Pillai Vs. Navaneethammal & Anr. (see here), it was held by the Supreme Court that a stamp paper, even if it is more than six months old, is valid to be used. Section 54 just bars taking refund after six months of purchase, but it does not restrict the use of such old stamp paper for an agreement. Thus, nothing prohibits you from using it even after years of its purchase. Law does not have any prescribed period of limitation for its validity.

You can make use of the ready reckoner in this regard to get a correct picture about it.