There is a large variance in "encounters per session" between each group for a variable number of reasons:
- Time alloted per session (Some people play 3 hours per session, some play twelve)
- Complexity of each combat encounter (Throwing some monsters in a blank terrain is faster than creating multiple complex interactive objects)
- Experience with the rules (More you play, less you need to consult rules on the fly, cutting time)
- Group size (small groups have less time between turns)
- Group composition (some classes are more direct like Rogue and Sorcerer, some like Shaman and Warlock are filled with minutiae)
- Group interest in "roleplaying" (some groups love to talk with each NPC and between themselves. Some like to jump from combat to combat)
- Etc, etc, etc...
The only recommendation in the DMG (pg 121) is:
The experience point numbers in the game are built so that characters complete eight to ten encounters for every level they gain. In practice, that’s six to eight encounters, one major quest, and one minor quest per character in the party.
That is an average per level, but not per session. Exactly because the rhythm per session is table-specific.
Since D&D 4e is build on the premise that people like tactical encounters (in contrast with other editions, who usually favor "exploration" over "combat"), it gives you a lot of tools to make combat very interesting. That's why you have a lot of combat rules. There is no problem with a combat encounter lasting one hour or two, as long as everyone is enjoying that hour-long combat. If you think the combat is going too slow for your group's interest, you can try another system that fit better your playstyle.
Now, in my personal opinion, your group size is within the limits, but leaning on the too large side (I would recommend 4 for starter DMs, but the game support 4-6 a a starting number), playing for too little time (I usually recommend 4 to 6 hours per session), and with very spaced sessions (Best interval is weekly or bi-weekly). This time interval make it hard for you to keep grasp of the rules between sessions, and sometimes even forget about the game's storyline and have to be constantly reminded ("Who is this guy we are talking too? Oh, right..." Think of it like if you play a new videogame for two hours, and then only pick it up to play again in eight weeks. You have to be reminded of story and mechanic as well.)
In the end, what matters is if your group had fun. RPG is in general not about how fast you reach a destination, but what you do in the voyage. As long as everyone is having fun, keep your own rhythm and enjoy the game.