For how long is it okay to leave a dog in the house alone? Will it feel abandoned if I am at work all day long? The breed is Maltese, age - 3 years. I rescued it but I love alone and have a full-time job.
If your dog knows that it's loved, then it should not feel abandoned. And if the dog has good bladder control, it should not be a problem to leave it alone while you're working. If bladder control is a problem, then we can work on solutions with you.
To be frank I have no idea if he has good bladder control. I haven't yet had time to take him to a vet. He's been living with me for less than a week. So far there's been no bladder issues. I guess it's a good sign?

One more question. How do I make sure he knows he's loved?
No evidence of bladder issues is a good sign.

To make sure that your dog knows he's loved, pet him, play with him, teach him tricks, walk him, praise him, give him treats when he earns them, etc. Basically just be a good pet parent and spend quality time with your dog, and he will know.