Can I ask my husband to give me time to accept this before he goes ahead with his plan?

Asking for time to process your feelings is a reasonable request, even though it does not put the decision entirely in your hands. You can say calmly: ‘I need a little time to accept this; please give me that space.’ A loving husband will appreciate this request because it protects the harmony of the home. Use that time to calm your heart, pray and think clearly, not merely to delay. Understand that in terms of the ruling, polygyny does not depend on your permission, but the manner and timing of carrying it out can be discussed with kindness (ihsan). If you feel rushed and pressured, say so. A careful discussion reduces hurt later on. See existing wife.